Fridge Horror: Just take a second to imagine what must have had to happen to the sweet, optimistic, puppy dog young Vimes to turn him into the bitter, self-loathing, apathetic alcoholic that we remember from Guards! Guards!. At least we know he eventually gets better.
Fridge Brilliance: In an earlier book Vetinari posits that he rather thinks he invented Vimes. In this book we see what he might mean: we see young Vetinari increasingly intrigued and impressed by Keel!Vimes, to the point he saves his life on several occasions and eventually fights in his name. He may notice the special attention Keel!Vimes pays to young lance-constable Vimes. He remembers Vimes' name, keeps an eye on him. So years later, when Vetinari's the Patrician and has to choose a new Captain of the Night Watch, he takes a punt on Vimes. We know, after all, that Vimes' promotion was Vetinari's personal decision (Vimes interprets it as a sick joke). By this time Vetinari may even have some idea of the time-travel hijinks. He is VERY clever, and he's certainly worked it out by the end of Night Watch. Regardless of whether he thinks of the late John Keel as Vimes' mentor of future self, he recognises when Vimes begins to live up to Keel, and is pleased with how he, Vetinari, has manipulated events.
There is lot of play around visibility and invisibility in Vetinari's storyline (and in the novel in general, actually). Vetinari 'gets' invisibility more than the other assassins. He realises that wearing black-only clothes, as is the Guild policy, makes one MORE visible, and that animal-inspired colour and pattern are more effective at creating camouflage. Black actually makes his fellow assassins stand out like a sore thumb. Fast-forward to his Patricianship, and he is noted for ONLY wearing black clothes. As an assassin he wanted invisibility. As the Patrician he wants to be highly visible. Genius.
Plus, it justifies how he was able to observe the final events of Vimes' sojourn in the past without Vimes noticing he was there.
Fridge Brilliance: By the end of the book, Young Sam Vimes had become a surrogate son to Sam Vimes and the audience was dreading the moment that we would have to part with him. So it is a particularly brilliant and symbolic move to have Sam Vimes knock down his younger and idealistic self in order to ensure his future. And then you realize that young Sam Vimes woke up to see his mates and mentor dead, that the revolution was rotten to the core, that low grade evil officers like Knock and Quirke emerged to become his boss and it hits you that the Young and Idealistic Sam Vimes was gone forever. So it's especially brilliant that when Sam Vimes returns to the present, back to the wife and child that represent his future, that his new born son was named Sam.
In a rare case of Fridge Heartwarming, consider that Vimes and his Treacle Mine Road allies have spent the last thirty years feeling somber every 25th of May. But from now on, those who know Vimes well won't see it solely as a day to be mourned, because it's also Young Sam's birthday.