I believe the plan with Luke was to give him a taste of the dark side. If he used it to strike down Vader, he would be drawn to it, like a drug, and he would keep using it, eventually falling completely.
And I remember always believing that Vader's allegiance was absolute. He was too far gone down the dark path to change now ("It is too late for me, son. The Emperor will show you the true nature of the force," even though that line has a bit of dichotomy, admitting he went down the wrong path but then immediately claiming that his way was the true way.), and that the Emperor would lash out against betrayal. Which is also kinda weird considering that Palpatine makes no bones about the fact that he's trying to replace Vader, so you would think that Vader would choose a death with dignity rather than purposefully being a pawn. Eh, the whole thing is weird if you ask me and yet another oddity about ROTJ.
But that's compounded by the PT and Lucas's current assertions. It always seemed there was some sort of arc in the OT. Once Luke tugged at the old heartstrings, Vader began to question his lifestyle and allegiances and became unsure of whether or not he wanted to be killed to make room for Luke. But the PT pretty much makes it that Vader was never loyal to Palpatine from the moment he joined the dark side. After all, he was just doing it for love, and Vader even says, "We can overthrow him," and, just like in ESB, I can respect that if it's just Vader's MO to BS about overthrowing the Emperor in order to bring others to their side. In fact, it's a rather cool, sadistic way to do so. "I hate the Emperor!" "Well, so do I. In fact, if you join me, we can overthrow him!" Five years later. "Wow, I love the Emperor." "So do I." There's a clear manipulation there, where Vader uses people's hatred of the Emperor to bring them to the Emperor's side, similar to the stories where Jedi attempt to control the dark side for good but end up being consumed by it. But Lucas screws that up too by saying that Vader was literally always trying to overthrow the Emperor, and this is his first time to attempt it. I just never bought that Vader was really trying to overthrow or even distance himself from his master. Like I said in my earlier post, he even turned down chances to do so. And if Vader was discontent with his status, why did he continue to serve for a generation after his sole motivation disappeared. Apparently The Force Unleashed's plot has to do with another of Vader's wacky attempts to dethrone the Emperor, but I've never played it, nor do I intend to, so who cares?
And considering how many apprentices Palpatine goes through in the PT, it does seem bizarre that Vader would stick around as long as he does, especially if Sith are as backstabbing as advertised. Granted, getting Vader as his apprentice was supposedly his main motivation in this ridiculously convoluted plan. And I'm not sure if it's considered canon or not that either Palpatine or Plagueis had any role in Anakin's conception, but if that is the case, why not just snatch Anakin up before Qui-Gonn appears, train him as a Sith from the time he's a child, and there you go! But now I'm just getting off topic.
But after Palpatine goes through all that trouble to get Anakin to become Vader, Vader immediately gets all screwed up and encased in his suit, which, according to Lucas logic, depowers him considerably (he's lost so many midichlorians after all!), which I think was just some bullshit retcon to explain the "lame" lightsaber fight in Star Wars, which, in itself is contradicted by Dooku and Grievous (good job, George). Suddenly "The Emperor's prize (is) damaged!" Palpatine must have felt pretty stupid to have gone through decades' worth of trouble just to get a half-man cybertron. Knowing his temper, it would have made more sense if he'd just discarded Vader and made another new apprentice rather than keeping this weakling around for twenty years. Again, this is going by Lucas logic. Vader always seemed plenty badass to me.
In other words, the whole "Must obey my master," shtick only really works if Vader is not actively trying to overthrow the Emperor. After all, that would mean he was seriously trying to recruit Luke in ESB only to turn down Luke accepting that offer in ROTJ. So in conclusion... don't watch the prequels. They give you a headache if you think about them too much.
