Whenever you win a battle against the same race, you're often given the option to replenish your own forces, ostensibly by absorbing enemy captives into your own army. As the Empire, this involves press-ganging the prisoners into service. As the Vampire Counts, this involves overriding your defeated rival's control over his zombies. As the Greenskins... you eat the captives, and this somehow replaces dead members of your army.
It has always been Total War convention that a certain amount of your casualties are simply wounded beyond combat effectiveness, rather than all dead. Having access to medical services (the Barber-Surgeon follower for the Empire, Apothecary building chain for Bretonnia, etc) gives a substantial boost to your troop replenishment to represent getting those wounded back on their feet quickly and efficiently. It was my assumption that the Orcs eating their captives accelerated their healing process and gets dem 'urtin boyz bak to da WAAAGH!
While most factions use gold to purchase things, the Vampire Counts and Warriors of Chaos instead use abstract and supernatural currency. Not terribly strange, considering the factions in question, but then you visit the diplomacy screen and discover that it's also possible to trade with other factions using 'dark magic' and 'favor' in place of gold. Karl Franz can accept intangible magic energy as a gift, and the Greenskins can try to bribe you with the favor of the Chaos Gods.
The factions could possibly, with emphasis on possibly, be paying the relevant faction using the abstract currency. Karl Franz could be accepting dark magical artifacts, Norscan tribes could be demanding gold tribute in order to back off. One cannot explain the Orcs ever managing to muster the favor of the Dark Gods (except for Khorne, maybe), or how any faction would be willing to do the evil rituals necessary and still be untainted. We may have to fall back to the MST3K Mantra on this one.
Could be some sort of exchange thing. The Vampires/Chaos factions could be using dark magic/divine favour to find or create gold while in the reverse they could be sacrificing gold (or things purchased with it) in rituals.
Archaon's role in the lore always struck me as an oxymoron. Although a powerful warrior in his own right, his true power is his ability to unite the forces of Chaos under one banner. But it's Chaos, the literal definition of randomness, disorganization, and spontaneity. He essentially controls the very antithesis of control. But when you get further into the lore, it's revealed that while he's outwardly serving them, Archaon's real motivation is to spite the Chaos gods, to deny them the world he was born into and revealing all their philosophies as a sham. To achieve this by enforcing a measure of order upon Chaos itself would be a delicious irony, and make it all the more bitter for the Chaos gods to swallow.
There's a difference between Chaos and chaos. The concept of chaos is a lack of order while (capital C) Chaos is a malevolent force emanating from another plane of reality. While the forces of Chaos are often chaotic they don't have to be (Nurgle's lot are fairly predictable) and you can be chaotic without serving Chaos (see the Orcs). As such there is no contradiction in unifying and controlling the forces of Chaos, it's just really difficult 'cos they all hate each other. As for the other thing Archaon is kidding himself; he's not stupid but he's not outsmarting the gods. His victory would plunge the world into a realm of Chaos which is exactly what they want.
Some wonder why the Vampire Counts speak in British accents rather than the Eastern European accents that some might expect them to have. Seeing that Sylvania was once part of the Empire, and the population of the Empire speak in British accents, it would make sense why.
Actually the people of the Empire speak Reikspiel, which is supposed to be similar to German, but it is very difficult to do English translations of Germanic accents without it sounding ridiculous, so most adaptations of Warhammer Fantasy give the Empire characters English accents.
With The Grim and the Grave we get Vlad von Carstein, the first and oldest of the bloodline, who does in fact have a strong Hungarian accent. Vlad is a foreigner that came to Sylvania back when it was an imperial province, and so wouldn't have the same accent as his Empire-born descendants.
This explanation no longer really makes sense with the release of Isabella von Carstein, who speaks in a similar accent as her husband, even though Isabella was previously a von Drak and a native to Sylvania. It in fact hints that Sylvania's people used to have much more Eastern European sounding accents, over five centuries ago. Either that, or she simply adopted her husband's accent over time.
Meanwhile, the Norscan Marauders all speak in the same vague Dutch accent that the Norse and Germanic tribes did in Total War: Attila, CA's previous offering. Makes sense, since they're essentially a Fantasy Counterpart Culture to the Vikings. But the Chaos Warrior and Chaos Chosen units also speak with the same vague accents, which is a bit strange, but then makes sense when you realise the armoured Chaos Warriors are simply the elite warriors and champions of the Norse tribes. Their accents are simply an indicator of their tribal origins.
Even Archaon, who's nominally an Imperial, speaks with the same vaguely Germanic accent the rest of his army uses. Why? Because he's not an ethnic Imperial, his father was a Norscan, and living for centuries in the North surrounded by Norscan warriors would have naturally caused him to lose his accent.
The Vampire Counts' mid and late tier buildings are quite expensive, making sacking enemy cities for extra resources especially useful. It makes sense when you remember that Vampires must regularly drink blood to keep themselves going.
Actually not entirely true, as in the Warhammer universe, the older a vampire gets, the less blood it actually needs to drink to satiate themselves. An elder vampire, like Vlad for example, can actually live on just feeding once a year if need be. Not comfortably, but he won't go insane from the hunger. That actually explains why Vlad could fall in love with Isabella (when she was human) since he didn't constantly view her as prey.
Why do Chaos armies experience attrition when parked too closely together? There's only so much Chaotic favor to go around, so inevitably the two armies will start competing for resources and space, often violently. Rival tribes might be tempted to settle old scores, and champions will start dueling in order to advance themselves in the eyes of their gods. Evil IsNotOne Big Happy Family.
Why can't the Vampire Counts use Global Recruitment to reinforce their armies? Because their undead minions are so mindless, they can't be expected to competently march miles across the globe just to join with their masters. Furthermore, without a sufficiently-powerful necromancer nearby to invigorate and direct them, even the mightiest undead army will eventually crumble to dust.
Empire Missions seem to be random incentives to pick sides in the Elector Count disputes and even external threats, occasionally. At first it seems like just the game giving you some suggestions and rewarding you for following them... or, perhaps, the various Electors are trying to sway Karl Franz's actions?
By this fridge brilliance, they're also trying to get Karl Franz killed. Fulfilling all of the missions will cause the Empire to be in a war with practically every province. These missions can try encouraging the Emperor to raid a Dwarf territory for money; yes, let's raid the faction that is not only supposed to be an ally, but will also record the raid as a grudge. This will cause that particular Dwarf faction to declare war on Karl Franz later on, in all likelihood.
Although it's not outright stated, but Bretonnia is strongly influenced by the Elves. Their cities are built on old Elven colonies, their architecture mimics their designs, many of their heirlooms and prized relics or of elven make, and in Warhammer: End Times it's revealed that The Lady of the Lake which all Bretonnians worship is actually Lileath, the elven goddess of dreams and fortune. So what are Grail Knights exactly? Beings who are faster, stronger, and more agile than your average human who never age? Who were directly granted these characteristics by a bonafide elven goddess? They're essentially former humans who have been transformed into lesser elves.
This would also mean, ironically, that the Grail Knight are all technically mutants in thrall to a warp entity (albeit a relatively benevolent one). Not So Different from the forces of chaos.
The Bretonnian nobility's contempt for the peasantry may well be a foreign influence. The Elvish overtones in their culture creates an awkward social dilemma, in that the Elves themselves are utterly disdainful of humanity at best. The High Elves consider humans to be witless primates who are almost more trouble than their worth, the Wood Elves treat them with utter indifference (whenever they're not hunting us as prey) and the less said about Dark Elves, the better. Each of these sentiments are reflected in the Bretonian nobility's treatment of their subjects. The peasants are essentially slaves who are exploited to death for their lord's benefit (like the Dark Elves, minus the wanton depravity). The Lords are obligated to protect the peasants from outside threats, so long as the serfs know their place, don't talk back, and take the brunt of the casualties (High Elves). Finally, barring any outside threats, the peasants are utterly neglected by uncaring, distant lords who do everything they can to distance themselves from the rabble (like the aloof and isolationist Wood Elves). We see in Bretonnia a sort of faux-elven nation where the nobility adapt much of the same customs and aesthetics, but also the elitist contempt for the more "mundane" people of the world.
In the real world, a common title for the elves in the medieval period was Our Lords and Ladies. The implied connections between "The Fair Folk" and the nobility are prevalent even in our own myths and legends.
During the Vampire Counts debut trailer, the Witch Hunter and his men are surrounded and attacked by Mannfred von Carstein's undead minions. The Witch Hunter's men are slaughtered until only he remains. He stops fighting and seems resigned to his inevitable demise, but he is not actually killed by the time the trailer ends. Seeing that Mannfred has a habit of torturing his victims, it is likely that Mannfred intends to not give the Witch Hunter the mercy of death just yet.
It's implied that this witchhunter is Gunther Stahlberg, who was sent to investigate the growing corruption of Sylvania by Volkmar the Grim. Two years later, Mannfred will officially announce Sylvania's succession from the Empire by dropping Gunther's skinned corpse on the floor during the Conclave of State (the annual meeting of the Elector Counts).
The flavor text for welcoming refugees and other people seeking safety during the Migration event suggest that your factions leaders plan on conscripting these people into the army later. Imagine how many people left their homes to escape monsters like the Orcs, Undead, or Chaos, only to be forced to take up arms and be sent to face them anyway.
It's even worse if you're playing as the Vampire Counts, whose armies are entirely made of undead creatures. It's easy to imagine the von Carsteins welcoming desperate refugees into Sylvania with promises of safety, only to start systematically "conscripting" them behind the scenes. The idea of anyone being hopeless or desperate enough to ask the Vampire Counts for protection is terrible by itself.
And on the flipside, if you send them on their way... well, if you're one of the last few factions left, that could mean abandoning them to their doom as they are forced to wander, vulnerable prey for any of the Old World's wandering monsters.
Several random events pertain to disasters happening within your realm, such as manticores swooping down from the mountains or Dark Elves raiding coastal settlements for slaves. The consequences of these events simply happen, with no input from the player, suggesting that your faction is abandoning its people to the ravages of the Crapsack World in order to focus on bigger matters.
Whenever you defeat a human faction in battle as Bretonnia, you can offer the survivors a chance to join your army. Most of these guys just go from being in a well-paid professional army into being cannon fodders in a conscripted peasant army, and if they ever retire, they will retire to the oppression of Bretonnia instead of whatever country they come from. This is the rough equivalent going from serving in NATO to serving in North Korea.