First off, I think the writers completely forgot about Darth Vader’s motivation in Episode V. If you can recall, Darth Vader seemed to be misleading the Emperor into thinking that he would either turn Luke or kill him. But instead, we find in the famous scene where he tells Luke he’s his father, that Vader was actually plotting to get Luke to kill the Emperor for him. Come Episode VI, this plot line is completely abandoned, and I think that’s a real shame. I think it would make the movie 10 times more interesting if Vader was constantly bothering Luke, trying to get him to turn to the dark side so he can kill the Emperor. And at the same time, everyone on the light side can be telling Luke the same thing; kill the Emperor! We can then have Luke plan to turn his father back to the light side, and Vader will ultimately become the one who destroys the Emperor. After all, this was Vader’s problem. He had been living as a pawn of the Emperor for decades. It would be ultimately cowardly of him to get his son to get him out of this mess. And it is his Son that ultimately redeems him. So firstly, the movie should be more about Anakin’s relationship with Palpatine, and how it has soured.
While this sounds interesting as an idea, I think it plays out better in the movie. First off, based on purely what we saw in TESB is Vader’s one line about Luke killing the Emperor and ruling the galaxy together. Fine, but we do not know if he was telling the truth. Just moments ago he said to the Emperor that Luke would be their ally and he would join them or die. So Vader very well could just be saying anything to lure him to the dark side to serve his master.
Secondly, in ROTJ I think it would be harder to get more emotion out of the situation if Vader was basically on Luke’s side from the get-go. It plays out better when he must obey his master to the end where it kills them both. Otherwise it would’ve felt like “well if you didn’t kill him, I guess I had to do it myself” and then you’re just thinking why didn’t he do it years ago.