******WARNING: LONG POST; SPOILERS SPOILERS SPOILERS AHOY!******* =D
Like the thought I had when Ridcully walks up to Vimes in Jingo and addresses him with his first name. To be fair, Ridcully is the sort of person to address anyone with their first names, but it still conjures up some fun ideas:
Like them gravitating towards each other at official gatherings because they actually enjoy each other's company.
Hmm, the only person I've seen who calls Vimes "Sam" (other than Ridcully) is Sybil. Everyone else calls him one of these names:
- "Sir" (Carrot, Angua, Nobby, Willikins the Butler)
- "Commander" (most of the rest of the way watch)
- "Vimes" (mostly the nobility, especially Lord Rust)
- "Sir Samuel" (Vetinari and Lady Margolotta)
- "Mister Vimes" (Colon and a select few)
- "Your Blackboard Monitorship" (some dwarfs)
- Something unprintable (most of the grags)
Plain old "Sam" doesn't seem to crop up, except for Sybil (and Ridcully, this one time).
I don't think Vimes actually
enjoys Ridcully's company socially - he's too suspicious - but he definitely has nothing against him or any of the other wizards. As Pterry points out, Vimes likes the wizards because they don't commit crimes that cause him any headaches. They might blow up the universe or summon eldritch horrors, but it doesn't lead to paperwork.
Or maybe Mustrum is a friend of Sybil's and he comes to visit every now and then.
A definite possibility. Sybil is more known for hosting the nobility of A-M, though.
I think it's more likely that Sybil knows Mustrum Ridcully through Vetinari. Sybil hobnobs with the Patrician quite a bit, and so does Ridcully (see, e.g. the start of "Interesting Times", where Ridcully is asked to find "The Great Wizzard").

Maybe they became acquainted that way.
Or the idea of Ridcully and Vimes having an amiable game of snooker or cards, as they have a private laugh at someone else's expense.
I find it hard to imagine Vimes playing snooker in UU. He'd find it hard to make any shots around Ridcully's piles of paperwork on the snooker table -- not to mention the balls exploding or turning into doves.
We rarely ever see Vimes paying anyone a social call (except when he pays them to Sybil in
Guards! Guards!). He's usually all business.
Idea for the Bucket: There's an unwritten rule that you don't sit at the Commander's table unless you are one of the people who address him with "Mister Vimes". Took a while for Vimes himself to notice it and he begrudgingly had to accept that as Commander, he's not part of "the lads" anymore.
I'm not sure there even
is a Commander's table at the Bucket. We rarely see Vimes at the Bucket -- we assume he's there, but assumption isn't the same as truth. When the rest of the Watch are in the Bucket, we usually then cross-cut to a scene of Vimes at his desk, working on a case.
(An exception is when a bunch of thieves break into the Bucket, and desperately -- and very foolishly -- try to take Angua hostage. Just before they do, we see Vimes growl something like: "Won't somebody shut the bloody door?"
I love this scene, especially when someone tells Angua not to play with her food, and Carrot shoots them a look of pain, and Angua thinks that she doesn't mind comments like that, but she knows he doesn't like it ... so she'll have to leave A-M at some point).
To sum up, we rarely (if ever) see Sam Vimes 'off-duty'.
Guards! Guards! has a few moments like this -- when he leaves a pub at the start and tells the landlord where to stick his beer, or when he pays Lady Sybil a social call, or when the Watch goes off-duty to prepare for the new king's coronation etc.)
But after that, the only time Vimes is truly 'off-duty' is during
Men At Arms, when he is preparing to get married. And - oh yes - that
dreadful (for him) dinner party, when most of the nobility talk about how the trolls and the dwarfs are ruining Ankh-Morpork. It's so unbearable, he has to be sarcastic to the various Lords etc.

I love how Pterry skewers racism there.
I think we see Vimes 'off-duty' during
Feet of Clay, too - when he goes along to Dragon, King of Arms, only to find out that he has no official coat of arms because it's been
excritus ad altitudone (or something like that - 'expunged from the record books, anyway). It needles him, because although he never wanted a coat of arms, now Nobby is getting 'particularly fine one'.
NOBBY, of all people. The world is upside down!
Anyway, sorry to rant so much. Please feel free to ignore my ramblings
