What would make you think that Darth Vader would be resurrected in Episode VII? Darth Vader is no more. He reverted back to Anakin Skywalker at the end of Episode VI. Didn't you notice that ? If anything, I could see Luke somehow crossing over to the Dark Side and maybe becoming the new Vader. Like his father before him. The cave, remember his failure at the cave? He saw himself inside the helmet.