Once Vader finishs hunting down the jedi after ROTS, he's made himself obsolete. The regular troops fight all the battles. Imperial commanders command the troops. The Generals and Admirals command the cmmanders. The Grand Moff's (governors) command the generals and admirals. And the Grand Moff's answer to the Emperor, which probably isn't often needed. At least the Emperor strategises. He comes up with plans. He sets traps for the rebellion. He's the head of the galaxy. Vader is in command of his ship the Executor and that's it. As I said, he's obsolete. No one can defeat him in combat, and no one would try. So what else is he good for now that he's beaten all the enemies that only he is capable of defeating? Ever wonder why he was so eager to get into the dog fights at the battle of Yavin? because he's friggin' bored and he's a great pilot and we all know how Anakin hates to have his abilities go to waste. Its almost as if the Emperor simply gives him busy work. The rest of the Empire would probably throw a party if Vader died.