In Return of the Jedi, when Vader took Luke to the Emperor, did Vader know what was going on? Did he understand that Sidious was going to try to replace him? In light of what we see in the Episode 3, book and movie, there are many more explanations available to examine.


Yes, He Knew
Assuming that he knew, why did he take Luke, or rather what did he think, or hope, that the outcome would be? One could say that he took Luke, trusting in his own strength to defeat Luke and keep his role as apprentice. However, we must also consider the possibility that he was hoping Luke could defeat the Emperor as the Emperor said he could and then Vader and Luke would be master and apprentice, but Vader could not allow Luke to strike Sidious down because of his ties to Sidious, to Palpatine.

No, He Did Not Know
If he didn't know, then what did he think the outcome would be? Did he assume that Luke would become like Mara Jade? If, in fact, he didn't know, then it's likely that his trust of his mentor was so great that it did not even occur to him that his master, his mentor, his "friend" would try to replace him.

My Opinion
I believe that it never occured to Vader that Sidious would seek to replace him. Vader trusted Sidious, betrayed and destroyed the Jedi Order for Sidious, killed his best friend for Sidious, tried to kill his son for Sidious, and in the end he learned that "treachery is the way of the Sith," but also "no gratitude is to be expected from the wicked." He didn't think that Sidious would seek to replace him. That's why he took Luke: he took Luke because he was sure that Sidious would convert Luke and keep him and Vader. When he learned that that wasn't going to happen, he destroyed Sidious.