I agree. Vimes has been a favourite character of mine for some years, so I'm afraid this will be a little long (sorry!)
I think Vimes's development began when the Watch started to become bigger, after the conclusion of MAA. (I haven't read MAA in some time, and I know G!G! much better, so most of my comments will come from G!G!).
in G!G!, he seemed like an affectionate parody of the noir detectives like Philip Marlowe and Sam Spade (though, of course, neither Marlowe nor Spade ever had to deal with someone like Nobby).
The reason I compare Vimes in G!G! with Marlowe or Spade is that not only is he the protagonist, but he does most of his detective work himself. The police in Chandler's and Hammett's books are supporting characters.
In G!G!, also, Vimes attaches some importance to clues (like the foot print), unlike Vimes in, say, FOC - where he almost dismisses the vital clue of the broken match-stick, because it doesn't make sense at the time. ("Oh," said Vimes sourly. "A clue".)
By the end of MAA and the start of FOC, the Watch is obviously a growing force in A-M, and Vimes a major player. He's enough of one to give Vetinari a nasty shock at the end of FOC; Vetinari doesn't show it, but he comes across as being irritated by Vimes's axe in the table.
Speaking of which, the relationship between Vimes and Vetinari is complex, in that Vetinari clearly knows much more than Vimes does but prefers to allow Vimes considerable leeway. This is understood by the fact that if he didn't, there would be no story. I believe he is also secretly amused by the fact that Vimes continues to annoy the people who deserve to be annoyed, like Lord Rust (in MAA and "Jingo").
TFE,
Thud! and
Snuff (and, to a lesser extent, MR and
Raising Steam) mark a departure for Vimes, in that we see him in action outside A-M. (We still see him in action in A-M throughout the Moist books - some Tiffany books, IIRC). I'm not sure if he becomes more or less irascible, but he never lets go of his belief in the law and his "sticking up for the underdog" mentality, both of which are very endearing.
"Night Watch" is an oddity because of the time element, but it's a great book and one of my personal favourites. (It's also one of my only signed copies, which obviously makes it more of a favourite).

The element of danger for Vimes here is certainly greater - or at least as great - as earlier books.
I don't know if there is more than one "turning point" in Vimes's character development, but certainly his marriage to Sybil (MAA) and the birth of his son (in NW) count. The danger level increases in
Thud!, so Vimes finds a weapon with which to fight it (i.e. the Summoning Dark tattoo).
My expectation was that in
Snuff, the danger level would increase again to compensate for Vimes's increased strength - and I can't imagine if it did, but it's been years and years since I read
Snuff. However, the tattoo certainly comes in handy during
Raising Steam.
Anyway, I could go on and on, and I'm sure I've probably forgotten a few things - but I'll give someone else a go.
