I just can't help but apply my own ridiculous dialogue to that scene. Vader: "If he could be turned, and I replace you, then he could be a powerful apprentice." Palpatine: "Yes... Yes... He would make a better apprentice than you. Will you be able to capture him and bring him to me so that I can begin his training as your replacement?" Vader: "He will join me against you, or he'll die, my master." That's essentially how the conversation goes, since any comment about Luke joining Vader and Palpatine with the Rule of Two still in play essentially means that by propositioning Palpatine with the offer to find and turn Luke, Vader is actually openly threatening Palpatine. Them be fighting words. But instead of Palpatine being like "hey! **** off, traitor! I'm going to kill you and turn your snot nosed punk of a son, and together we're going to dance on Padme's grave!" he says (paraphrased), "Go find Luke and I trust you to bring him back here, and I've just got to say that it's totally cool of you to take one for the team and lay down your life so that I can have a better apprentice, I just want to stress that this isn't personal." And then fast forward to ROTJ and Vader tells Luke that Palpatine will be his new master and teach him the ways of the Dark Side. And Vader stresses that he MUST obey his master. Put into context with the Rule of Two, this just makes Vader look like he's giving up. That he's lost, he knows he's going to die, but that he's willing to die at Luke's hand, if it means that Luke will be safe as Palpatine's apprentice.