The entire "send prawns by clacks" conversation is a Stealth Pun
Specifically, using an advanced communication system to send prawns... pr0ns... PORN.
Charlie is not only Vetinari's doppelganger but in fact the Anthropomorphic Personification of all the characteristics Vetinari left behind to become what he is now
- That, my dear friend, is BRILLIANT.
Some of the members of the Committee to Un-Elect the Patrician are actually working for Vetinari.
It's mentioned in other books that many people have tried to depose Vetinari, and that Vetinari was even behind a few of these attempts. It would be quite fitting for a master of improvisational scheming like him to keep tabs on a big conspiracy like the one Lord de Worde was running without de Worde knowing.
- Which is why the conspirators' chairs were all positioned so the occupants couldn't see each other: Lord de Worde presumably suspected as much, and arranged things accordingly.
Mr Tulip is Borogravian.
Very late in the book, it's explained that he got interested in art as a little boy while hiding from soldiers ("Ours or theirs, it didn't matter when the war had gone on this long") in a snow-bound church, which contained the only beautiful things he'd ever seen. Zlobenia is Borogravia's perennial enemy and, in Monstrous Regiment, not much mention is made of Zlobenia being ultra-Orthodox like Borogravia. Eternal war + church + cold = Borogravia.
Mr Tulip is Uberwaldian.
Moist von Lipwig is Uberwaldian, and his family are members of the Plain Potato Church (as opposed to the Orthodox Potato Church).
- I get the feeling that line was Moist's jab at their religious beliefs, not the actual religion. 'Sides, he'd have learned to deal with vampires, werewolves, and zombies if he vas from Ubervalt.
Why Vetinari tries to advise William about family issues
For no particularly obvious reason, when Vetinari first shows up at the newspaper office, he tells William to write to his father more often and says how sad it is when families don't get along. Possibly this is because William's brother died in the brief non-war in Jingo
. That bit in Jingo
where Vetinari actually gets kind of emotional (and knocks over his chair!) when he's arguing with Vimes shows that he thinks the non-war had pretty much the best possible outcome but he nonetheless takes it seriously that there were consequences. Maybe he tries to help William patch things up with Lord de Worde because he feels responsible for his involvement in William's brother's death, even if the little-bit-of-a-not-really-a-war was unavoidable.
Alternately, he suspected Lord de Worde was plotting against him and hoped William would write to his dad like "hey, what's up?" and his dad would be all, "not much, just plotting against the Patrician!" and William could report on it. Who knows.