Vader's fate could have been like Dooku's
We see in RoTS that in order to have Anakin as his apprentice, Sidious has to remove dooku, as the Sith only operates in two - one master and one apprentice. Anakin (the would-be apprentice) was asked to kill off dooku.
In RoTJ, Sidious wanted to turn Luke to the dark side.
EMPEROR: "Now..fulfil your destiny and take your father's place at my side!"
Didn't he just asked luke to kill Vader? When he said "fulfil your destiny" it was as good as saying 'strike your father down"..Vader must have been blindly loyal by bringing luke before the Emperor,it was as though they were discussing the removal of vader openly.
Surely Vader knew about the Sith's Rule of Two. It is my belief that when he heard the quote above by his master, he remembered Dooku dying in his hands,and finally it dawned on him that he was to die in order for luke to become a Sith apprentice.That is why, in addition to luke's appeals, he made up his mind to kill sidious once and for all.
If not, Vader's fate could have been the same as Dooku - killed by the would-be apprentice, Luke Skywalker this time round.