I like all the usual round of suggestions for Vimes: Hugh Laurie is clearly a physical match, and can play cynical and dry very well. Ben Miller looks right to me too. Someone somewhere suggested Jack Dee, which works for me.
But ultimately I think all of these actors would fail in a Watch TV series (at least the sort I'd want to see). They'd give fine performances, but not the right kind of performance, to my mind. What you really need is an actor with a particular quality - soulfulness - that draws the viewer in.
When I was watching the existing Discworld adaptations (I rewatched them recently), I was trying to put my finger on why they didn't really work for me. There's a few reasons (the lack of Pratchett's narrative voice is a problem, the tone is too broad etc). But one of the problems was the lack of depth in the character. Richard Coyle, for instnce, gave a decent read of Moist Von Lipwig, but divested of his inner voice, and hamming the whole thing up rather, the character wasn't sympathetic in the way Moist is on the page.
The trouble with actors like Hugh Laurie, to my mind, for the part, is that they'll give a performance that's watchable and funny, but not draw the viewer into the inner life of Vimes.
My current thoughts on this one, therefore, are: John Simm. He looks little like my mental Vimes, other than he's a skinny bloke of about the right age for Vimes-at-the-start. But Simm proved in Life On Mars that he can give a performance that takes you past the fantasy/high concept/camp/comedy stuff and makes you absolutely believe in his character. Look, see: http://nestofstraightlines.tumblr.com/image/39481787979
That works, right? Right?