Yeah, what Chaltub said. Basically, before the dialogue change, I assumed the Emperor saw Luke as the next in line. Then after the dialogue change, that still stood, but my belief that Vader didn't know about his son was changed. Now, after ROTS, the story plays out, Vader learns of his son from Palpatines admission in the transmission, and then tells Luke that the Emperor has foreseen that he can kill the Emperor. Not that the Emperor wishes it to be so, but that the Emperor even admits that Luke can do it. Thus, now Vader wants his son to join him to actually knock of the Emperor and not as some ploy to trick Luke to take him back to Palpatine.