Complex sentence: Vimes employs him because of his surgery methods, which Vimes considered to be considerably more advanced than most of Ankh-Morpork's doctors, in that most of his patients survive, and he doesn't use hot tar.
Split into simpler sentences: Vimes employs him because of his advanced surgery methods-well, more advanced than Ankh-Morpork's doctors. Most of his patients survive, and he doesn't use hot tar.