I think TFE is the first "Vimes" book, rather than "Watch" book. The Watch become secondary/support characters to Vimes.
***********WARNING: SPOILERS************ =p
Hmm, I agree in a way ... *watches everyone waiting for the "but?"*
... but ("told you so!")

... Vimes in G!G! is still the protagonist. We see things mostly from his POV. Everyone contributes, especially Sybil and Carrot (and, to a lesser extent, Colon), but it's Vimes who investigates the mystery.
In MAA, Vimes takes a backseat to Carrot -- which is understandable, given that he's due to retire -- but Carrot is still not confident enough to take over (e.g. when Bjorn Hammerhock and Brother Beano are killed). Then Captain 'Mayonnaise' Quirke of the Day Watch takes over ............ oh dear.
I just realised.
Captain Quirke.
Oh,
no. *groan*

Terry, you devious ....... *shakes head*
FOC is the first book where Vimes comes into his own, I think. Carrot and Angua contribute by investigating the nature of golems, of course, and Cheery contributes by being the "forensics" department and testing various alchemical ingredients. But Vimes connects the dots (and Carrot connects some of them). Meanwhile, Colon puts the lid on Vimes's suspicions. It's certainly a "Watch" book, but Vimes is the central character.
Jingo is a "Watch" book, but we should probably expect that. Vimes doesn't investigate murders on his own -- Carrot/Angua and Colon/Nobby bring him the Clues(TM) -- but it's up to him to figure out what they mean. Vimes gets his Big Hero Scenes during the fire at the embassy and the arson at Mister Goriff's Curry Place (I forget the name of the shop, sorry). When the action shifts to Klatch, the book focuses more on Vimes than on Carrot -- although Carrot gets his own scene or two.
Is TFE is "Vimes" (not "Watch") book? I'm not sure. I see TFE as a book of two halves; the first half (before Vimes is incarcerated) is a "Watch" book, in that Vimes has direct help from Cheery and Sybil. (Carrot has his own scenes). The second half is a "Vimes alone" book, interspersed with Carrot scenes (and the "Colon making a mess" scenes).
Perhaps this book shows why it's not possible to write a Vimes-only book (i.e. Vimes the hero, where no-one else contributes). I once thought in those terms, mainly thanks to film critics who talk of films in terms of vehicles for this or that "star" - but without the contribution of many, many other people, that would be impossible. In the same way, without Carrot's and Angua's help here (and later, Detritus's, and earlier, Uncle Vanya's Trousers), Vimes would've been helpless, trouserless, and very dead.
Carrot gets the Big Scene of dealing with the fallout back home at A-M, but only because Vimes and Sybil are delayed. I'm sure this was intentional: if Vimes had to deal with Colon's mess, I can just imagine the shouting. Colon would probably be fired, at least -- and probably STP didn't want to deal with a Watch without Colon (who provides most of the broad comedy, after all).
Finally, with Night Watch, we see the shift into more dangerous territory (and philosophical discussions, e.g. the Nature of the Beast). Vimes gets most of the Big Damn Heroic Scenes here, so I suppose this is the most Vimes-ish book so far. Almost all the members of the Old Watch and New Watch still contribute along the way, but I can't think of a single Watch scene here that doesn't include Vimes in some way.
As raisindot points out, the Watch novels after Night Watch mark the transition into Super-Vimes, which is a different beast altogether. But even "Thud!" has the other members of the Watch contributing - Colon and Nobby at the Museum, Angua and Sally (in the Museum - I think? - and later at the Pink Pussycat Club). Carrot is conspicuous by his almost total absence. But in the end, Vimes does most of the work. "THAT ... IS ... NOT ... MY ... COW!" *grin*
"Snuff" strikes me as similar to "Thud!", except that it has a very Super-Vimes-in-the-Countryside vibe. When it started (and Vimes was talking walks, and Rust was explaining the rules of crockett, i.e. a blend of cricket and croquet), it struck me as a bucolic,
Dixon of Dock Green-type book. But that's fine: that was STP setting the mood, only to spring the trap when Vimes enters the pub and the three Toms (and Jethro the blacksmith) show up. (Won't spoil the rest).
Vimes takes a back-seat in the next few books that he appears in, e.g. MR, the Moist books (and yes - how could I forget - TLH). But speaking of TLH: this always struck me as a very "ensemble" book. Busby Berkeley without the dancing and singing (naturally), but very tightly structured and "choreographed", nevertheless. Everyone gets a moment to shine, which is why -- to me -- the ending is so bittersweet.
Anyway, I've been rambling for way too long. *blush* Sorry! I'll shut up now.

What do you think, hmm?