See, I disagree because Return of the Jedi ISN'T about Vader, again, despite what Lucas would have you think. It's about Luke and his internal struggles. His desire to bring his father back to the good side. Because of that, it's what we know about Luke that's important. We don't really need to know about Vader or the Emperor beyond what we get in ROTJ to understand their relationship, and, more importantly, Luke's relationship to both of them. In earlier drafts, Lucas did have more backstory and more scenes with Vader and the Emperor, but those were ultimately jettisoned because it needed to be more about Luke, and because those scenes tore down Vader as a villain and made him too sympathetic. If we know too much about Vader, and he's made sympathetic, then whether or not he will become good becomes a non-issue. Luke doesn't know for sure; therefore, we don't need to know until the moment that he does.