Maybe they explain that weird scene in Star Wars VI (where Luke goes to a place where he confronts a proxy of Darth Vader in which nothing is explained as to why it was even there,) which would probably require Darth Vader.
Actually, although that scene is not physically explained it DOES make sense. The Dark Side Cave, or The Cave of Evil is a cave that is 'tainted' with the dark side. It became tainted when the Jedi master Minch killed Bpfasshi Dark Jedi leader and the dark side aura kind of seeped into the cave.
But, that's not the point. Yoda sends Luke into the cave telling him:
That place… is strong with the dark side of the Force. A domain of evil it is. In you must go."
"What's in there?",Luke says
"Only what you take with you."
In a way, this scene is a sort of metaphor that shows that Luke's worst fear is becoming like Darth Vader. But, like you said, it is not fully explained and there are several theories as to what it could mean. The one I mentioned, being one of the most prominent. What he fought in the cave was not ACTUALLY Vader, or somebody sent by Vader, it was a vision of what Luke was afraid of becoming.
I hope that cleared things up and your enjoying the wait for Episode VII just as much as I am!