"it was clear in Jedi that the emperor wanted Luke to join him and Vader, either that or Vader was a big dope for not realizing he was being replaced"

I always thought it was a giveaway, this scenario, it's plainly obvious what was going to happen. Vader would off Luke and eliminate the last real threat (except for Leia, hyuk hyuk) or Luke would off Vader, and he'd have a young servant that wasn't in an iron lung. Or, Vader wanted to off the Emperor and work with Luke. It seems a simple enough concept to me.