What if we were to revisit that moment in Return of the Jedi when Luke decides to strike the Emperor down when they are on the Death Star II?
Let's just spitball here for it a bit and let yourself be dragged down the garden path, just this once...
As is commonly considered, the Emperor goaded Luke into raising his saber against him and in doing so this action caused Vader to intervene with his own saber and saving the Emperor's life.
This was all part of the Emperor's plan. He wanted Luke to give into the dark side of the Force, to help Luke let the hate flow through him. Once he had dispatched of his Father, Luke would have succumbed to Palpatine's influence as had Anakin some twenty odd years ago.
We know how this played out, Luke and Vader fought, Luke dealt Vader a mortal blow (or did the force lightning finish him off?), Palpatine attempted to kill Luke and Vader finally made up his mind about where is loyalties lay and saved Luke.
Great story, great ending to the movie trilogy.
What if when Vader stopped Luke's strike, it was not to save his master Palpatine but to stop Luke from taking that final path to the Dark Side?
We know that Luke appears to have been heading down that path.
He's started to wear black. He's slowly becoming part machine due to his daddy cutting of his arm and he forced choked a couple of Gammorean Guards at Jabba's Palace. So he's ripe for the turning right?
Had Vader noticed this change in his son?
Recall in Revenge of the Sith when young Anakin has Count Dooku down for the count. He is humiliated and beaten by Anakin. And what does Annie do? He excutes him with a double blade assault to the neck, mostly at Palpatine's bidding and thus turning him ever more so in the direction of the dark side of the Force.
What if Vader has all this in mind as Luke is about to strike?
We can argue that Vader's loyalty to the Emperor has been wavering throughout the whole of Jedi. That scene with Luke on Endor is arguably a key moment for Vader.
You may have noticed he doesn't respond to Luke when he suggests there is still good in his father. Vader is wavering in his loyalty to the Emperor even more than he was at the time of Empire when offered for him and Luke to team up and together rule the Galaxy.
So, Vader is wavering and he doesn't really want his son to become like him. So if he had of killed Palpatine with his first strike, that would have been Luke turning, just like Anakin did with Dooku.
Vader chose not to allow Luke to do this, and would rather fight it out a bit and see how things play out. Of course if he had of killed the Emperor, the turning to the dark side probably would only have happened if Vader chose to capitalize on it... but stay with us.
Luke seems to know this, and calls Vader out during the fight but manages to give away that Luke has a sister.
Does Vader know this is Leia?
We have no idea, however it's enough to goad Luke back into the fight.
Vader is wavering still but is suddenly balls deep in a fight to the death, a death he may have wanted for a long time since he was saved from being a crispy critter back on Mustpha by Palpatine.
Either way, by the time Luke beats him (how amazing is that swelling musical score then that happens!) Vader is nearly done.
Ultimately Vader chooses to let his son live but he took his sweet time doing it. Throughout that whole scene he could have gone either way but he really didn't want his son going to the Dark Side. He just wanted the team up.
Any ways, this has been a bit of a ramble, what do you think? Was Vader stopping Luke from falling to the dark side when he prevented Luke from striking down the Emperor?
Or maybe I'm over reaching. It's probably actually simply how it looks in the movie. Vader simply defends his master and it's not until he sees his son dying of a force lightning attack that he makes up his mind.
Extra for Experts
If you look really carefully at the very end of Jedi, post fight scene, Luke's costume is opened up, the flap's insides are white. This symbolises that Luke was always a Jedi like his father before him. Perhaps this casts doubt on all of the above, but the symbolism is clear. Luke was always the good guy.
Extra for Experts II
While we just said that Luke was always the good guy, we understand that an early idea Lucas had was for Luke to become the bad guy at the end, to walk of into the sun set having turned. How interesting a bookend to the Star Wars saga that would have been!