Corporal Nobby Nobbs would have to be played by Andy "Gollum" Serkis, though the images would have to be a little less benign. Only Rickman could be Vetinari - I recall seeing him play the Reverand Obadiah Slope in the BBC adaptation of Trollope's "The Warden" and "Barchester Towers", and have never seen anything quite so machiavellian.

Christopher Lee has voiced "Death" several times.

My other recommendations: Timothy West for Sergeant Fred Colon, and his wife Prunella Scales (with a bit of padding) for Lady Vimes (nee Ramkin).

See Internet Movie Database for cast list for "Hogfather", see what the industry has already come up with.