Plus, Vader isn't in the position to demand anything from Palpatine. I think one of the great ironies of the saga is that what drives Anakin over the edge is the desire to protect the people he cares about - "I will even learn to stop people from dying!" Palpatine implies that he can teach him that, and he never does! It's a total bait and switch, and Anakin is left to a life of indefinite servitude with 1/15th of a body and no friends or family. That's why you never do business with the bad guy.