I always felt that Vader was constantly plotting against Palpatine after a certain point in their relationship.
Consider the films: Vader reaches out to Luke in ESB to join forces with him. This could be (and I think largely has been) interpreted as the classic Sith mantra of betrayal of one another. But I believe Vader likely understood the presence of his son quite early, perhaps even before A New Hope. And knowing of the presence of another Skywalker reassured him that he could potentially defeat the Emperor one day.
In ROTJ, Vader ADMITS to sensing Luke aboard the imperial shuttle Han piloted to Endor, but he allowed it to pass rather than block their entry.
It is at this vital point that Vader's intentions should become clear to the viewer.
He knew that Luke had foolishly joined the mission where the Death Star would annihilate another rebel planet, or in this case a moon, and decided to act.
When he told the Emperor of his having sensed Luke, the Emperor responded that Vader's feelings may not be clear.
How did Vader sense Luke, but the Emperor failed to do so? I believe it's because Vader began to focus his energy on shielding Luke's force signature from Palpatine, allowing him to pass unchecked, but also to cause the Emperor to second-guess his own foresight.
The Emperor says Luke will show himself to Vader, and Vader will bring Luke to him.
Now, hear me out: this is EXACTLY WHAT VADER WANTED.
The rebels on the surface are given plenty of time to plot and strategize while Vader quietly picks up Luke and takes off with him. Vader didn't bother to slay the rebels? Hmm.
Luke & Vader return to the Death Star where Vader now knows he must allow Luke to channel the dark side far enough to help him kill Palpatine.
Vader pushes Luke to the edge, and Luke eventually does channel the dark side in combat (the look and feeling Luke has while hacking away at Vader's arm) and defeats Vader.
Vader is injured, but not fatally. He wanted Luke to be the one to kill Palpatine so he knew he could turn him.
However, here is another very pivotal moment.
The sight of the force lightning running through Luke is enough to jar Vader into channeling the light, and he realizes that he values Luke's life more than the Emperor's & his own at this point.
Seeing that Luke will not beat the Emperor alone, and knowing that it will take a sacrifice now to achieve his long-term goal, Vader flings the Emperor down the shaft, ending him once and for all.
I think Luke later completely understands the actions of his father, which allows him to view him in a more positive light than most rebels and Jedi generally did.
Fan theory over and out.