Matthew Lewis as Samuel Vimes:
Matt Smith as William de Worde:
I'm thinking someone along the lines of Tom Felton or Chord Overstreet for Moist von Lipwig, though it'd have to be when they weren't bleach blondes, because Moist, at least in my view, is dirty blonde. Leaning a little more towards the Tom persuasion, but that spots pretty much still wide open.
So, er, if you like Discworld, enjoy my view of young Sam and young William. And if you don't... well, first, enjoy this giant pic spam I guess, and then go read TPratch's books. Right now.