An interesting topic. I think I read somewhere that Luke had said that his father could have defeated the emperor at anytime during the last several years. He did not for the simple fact that Vader had absolutely no desire to rule over anything. He just wanted to destroy everything in the most vicious way possible after Padame's death. Palpatine was actually frustrated that Vader had not challenged him yet. And would even goad and tease him about it. Did he live up to his potential. Probably not. However Vader didaccomplished amazing feats. And he did live up to the title of the chosen one. It was Vader not Luke that brought an end to it.