Great villains from Uriah Heep to many of Blofeld's "number twos" (Largo in Thunderball, for instance) have answered to other people. What matters is where you place the dramatic focus.

Plus in the case of Vader, he clearly states his secret plan to overthrow the Emperor and rule the galaxy with his son. Hardly the plan of a lackey. Oh, and the Death Star does run to schedule. He arrives at the beginning of Return of the Jedi to make sure it does, then by the time the Emperor arrives it is. Just saying. ;)

Still, the idea of Vader having to fill out paperwork everytime he telekinetically strangles someone is rather hilarious.