If he'd struck Sidious down?
"I am unarmed, defenseless. Take your jedi weapon. Strike me down and your journey to the Dark Side will be complete..."
BS you're unarmed, Sidious, you're the most powerful Sith in history, full stop.
He would't have. He tried and Vader intervened.
So did Vader save Luke from falling?
In a way, yes. The pivotal scene was of course that after using all his anger to defeat Vader, Luke was confronted with the fate that awaited him if he continued that way.
To me, Luke is the true chosen one, not Anakin. "Once you start down the dark path, forever will it dominate your destiny", Yoda said. Luke was the first to get off that path. He was way more powerful than Vader. Vader followed by example.
Right. A case could be made for either or both. You could say that Vader is still the Chosen One, because his actions brought balance back to the force, but Luke is the one who embodied that balance.
I know. I never liked the 'chosen one' angle, it feels contrived and in the end, when it comes to bringing balance Vader is not making the major steps here. Yes, he chucks the Emperor down the reactor. But he was distracted by Luke who openly defied and resisted the Emperor. Luke is the real hero to me.
Vote Luke for The Chosen one!
Meh, couldn't care less. I hated ROTJ.
But RotS, though, man... crème de la crème.