well the answer would have to be both.....right?
One could say that Luke is part of the "prophecy". Another piece of the chain of events that set Anakin/Vader in motion to for fill the "prophecy". Also Luke = Anakin, as father and son they are really mostly the same. The force is strong in both.
If Luke wasn't around, who would pull Vader back to the light side? Who would convince Vader to kill Sidious to end the Sith, if not his son being slowly killed by Sidious force lightning.
Sure Vader might of been able to kill Sidious on his own accord but for very different reasons. One most likely to take his place and become emperor.
I'm surprised Vader never tried(?) or did it sooner. As we know the whole reason why Anakin turned to the dark side was to save Padme. Once Vader realised the empror wasnt going to do that, why stay his apprentice.
The only reason i've read on these forums is that in his current form as Vader he was to weak to take on the Emperor by himself.
I would like to think they both as father and son brought balance to the force and the galaxy. Sounds like something George Lucas would plan/like.
He stayed his apprentice because he was broken. He had nothing left to live for. The bigger question is why didn't he off himself? My guess is because he couldn't bring himself to do that either. He didn't care enough. Vader has said the thing that he hates the most is himself. Still, he did try to overthrow Sidious a few times. One with Galen Marek and then later by trying to turn Luke Skywalker. Other than that he simply didn't have the will and wasn't powerful enough to kill Sidious.
I believe at that point Vader only cared about serving the dark side of the force.