It just always reminds me that I never bought that Vader's motivation in the OT was to use Luke to betray Palpatine. Like Luke claims in one of the Zahn novels, I think Vader was just lying to Luke when he beckoned him to overthrow the Emperor in ESB. As we see in ROTJ, Vader seems totally incapable of defying Palpatine, even when Luke offers to do nearly exactly what Vader wanted him to do in the previous movie: run off together and defy the Emperor. So either that's more Lucas BS or those intentions just weren't conveyed very well at all. I like to believe it's more retconning because I honestly can't take ROTJ seriously if I'm laboring under the delusion that Vader's motivation is to defy the Emperor. Nothing in the movie supports this... well, you know, other than when he does, but saving his son's life is the motivation for it, not overthrowing him for its own sake.