I can't picture any actor for Vimes - I have such a strong picture in my head of him that it would take a long time to change that by watching someone play him who isn't exactly like my picture.
I can think of someone for Carrot though. I only know his first name was John, and I have only seen him act once, but...
He would need no makeup; he is about 6'3", bright ginger, built like a Greek god and possesses the stage when playing a bit part without even trying. He walks on and suddenly there is no one else there. Sound familiar? When I think of Carrot, I see that lad.
He was a pupil at King's School, Bruton, in Somerset. He would be about 26 or so now and I saw him in a school play when he was 17 or 18, just before he went off to spend a summer playing with the National Youth Theatre who had talent spotted him. I would love to know what happened to his career, or if he even continued in acting. I really should go back and ask the school for his name and see if they have any news. They keep track of all their old boys if they can.