I saw the Going Postal adaption on stage in with Stephen Briggs as Vetinari and sorry, but seeing anyone else in the role is like seeing someone try to impersonate Mark Hamill's Joker. He captures the tone perfectly, and there's always a slight edge of disappointment that no one challenges him enough to be truly interesting to him.
I suppose we should be grateful that it's not David Jason playing Moist.
Coyle certainly has a challenge ahead of him, one of the problems with adapting his stories was showcased in the version I saw was that Moist (Like Harry Potter) as seen is a terribly dull character, most of the drive for him comes from his INTERNAL reaction to things and events rather than anything external or noticeable on screen.
And they really need to make sure that Stanley and Groat are true to their origins as sort of pastiches of Graham Moffatt and Moore Marriott from the old Will Hay movies.