Yeah, but Vader's just sick of being the Emperor's puppet - he's power-hungry at this point and wants to rule on his own, not be the underling of someone he knows will never relinquish power. He knows, however, that he's not powerful enough on his own to stop the Emperor - but with Luke at his side, they may win. It has nothing to do with the "rule of two.

Yeah that's true, you can see it that way or (now that we know the PT "rule of two" ) that Vader is also trying to "save his skin" by turning Luke into his own apprentice before the Emperor does. I know that is probably not what Lucas had in mind when he shoot that scene in ESB, but now that we have to deal with the prequels I just try to find ways to live with them ;)

However I'm not against cutting that rule in a PT fan edit.