Fully agreed. Okay, going on some kind of AU where they end up living together:
who cooks normally?:
The staff. Although sometimes Sybil insists on cooking and Vimes and Vetinari will politely hide how horrible it is. (One time Vetinari made dinner and it was hideously complex and fiddly and small and had lots of avec. But against all expectations it was also delicious. Vimes is still sulking about it)
how often do they fight?:
Vimes and Vetinari pretty much all the time - Vimes isn’t happy unless he’s got something to push against. The other sides of the triangle hardly ever fight. When Vimes and Sybil have a fight it’s usually about how Sam should be more careful, but that’s rare. When Vetinari and Sybil fight, they both retreat into chilly formality, leaving Vimes unhappily in the middle like a confused puppy.
what do they do when they’re away from each other?
They’re possibly three of the busiest people in Ankh-Morpork; they don’t spend that much time together. Vimes does his coppering, Vetinari rules (heh) and Sybil wages her carefully-orchestrated social war campaign. It only makes them appreciate their time together
nicknames for each other?:
Sybil and Vimes “my dear” each other. Sybil and Vetinari are on first-name basis. Vimes calls Vetinari a variety of insulting names, and Vetinari always calls him Vimes - except on very very rare occasions, when he’s Sam.
who is more likely to pay for dinner?:
UM. I doubt anyone one dare to make those three pay for their meal. Although Vimes might insist on paying (although Sybil or Vetinari sneak back quietly afterwards to allow the poor restaurant-keeper to give the money back, because the idea of making Vetinari pay is one that terrifies everyone)
who steals the covers at night?:
Vimes, who grew up in a place where you either fought for your sheet or you slept without. Sometimes he uses his teeth.
what would they get each other for gifts?:
Vetinari would get Vimes somehow even more medals and honours, and Sybil laws on the Protection of Swamp Dragons. Sybil gives both her boys hand-knitted knobbly socks and scarves. Vimes gives Sybil flowers out of some confused sense of romanticism, and Vetinari the occasional interesting new weapon to study (and subsequently outlaw).
who remembers things?:
Sybil and Vetinari. And Vimes’ disorganiser.
who cusses more?:
Vimes, obviously. Sybil occasionally surprised Vimes and Vetinari with a well-placed curse when she’s feeling particularly annoyed. Vetinari never curses, much to Vimes’ suspicion.
what would they do if the other one was hurt?:
Roaring rampage of revenge for Vimes, understated controlled rampage of revenge for Vetinari, magnificent righteous rage for Sybil.
who kissed who first?:
Sybil kissed both Vimes and Vetinari first (Vimes after their second date when he was shuffling uneasily and trying to decide if it would be too forward; Vetinari when they were both still teenagers, and afterwards Sybil smiled gently and said maybe not yet?). Meanwhile Vetinari kissed Vimes (well after they first tentatively started their threeway-thing) after he’d done something stupidly brave and noble like standing up to the entire aristocracy or trying to arrest a country.
who made the first move/who started the relationship?:
Sybil, who decided to be sensible about this whole thing and frankly discuss it with both men, who were very surprised and more than a alittle uncomfortable.