Vader needed Luke to rule, because most people wouldn't be too fond of having a Big-Black Cape-Wearing-Part Human-Part Robot-Dude With a Creepy Breathing Apparatus being in charge of the Empire. Luke would have been the pretty face that everybody would have been OK with, but while secretly being evil with Vader as the puppet master.
This is also why Vader never was able to overthrow the Emperor himself, because he knew that the people would not get behind him at all.