I love Vader’s arc in ROTJ, I think it was done perfectly. I even rank ROTJ above ANH, personally.
The point of Vader’s “redemption” isn’t that he slowly became a good guy. That side was always there, he just suppressed it constantly. Throughout both ESB and ROTJ he has a soft spot for his son. But he just keeps lying to himself saying that he will kill him if he has to… but he never does. In fact he basically lets Luke beat him, since all of his conflict weakened him so much (Vader was way more powerful and skillful then Luke, but he was crippled by conflict). The point is that it was sudden on purpose because Vader saving Luke is supposed to be something he’s highly contemplative about but eventually gives into his good side. It’s an emotional choice at the conclusion of a long drawn out battle in which Vader is basically being forced into a corner to pick between the Sith and his son, and his good nature is essentially forced to rise again because he just can’t let his son die. And more importantly, it’s that after 23 years of basically being Satan’s right hand, continuously making the wrong choice, he finally did the right thing. The thing he knew to be right from the very beginning but couldn’t muster the strength to do. But in his final act he finally let go of his hatred and pain and chose to show compassion. The entire point is that he was evil right up until he made that decision. He doesn’t technically make up for everything he did because he’s not supposed to. Because he can’t.
"It really has to do with learning. Children teach you compassion. They teach you to love unconditionally. Anakin can’t be redeemed for all the pain and suffering he’s caused. He doesn’t right the wrongs, but he stops the horror. The end of the Saga is simply Anakin saying, ‘I care about this person, regardless of what it means to me. I will throw away everything that I have, everything that I have grown to love - primarily the Emperor - and throw away my life, to save this person. And I’m doing this because he has faith in me, loves me despite all the horrible things I’ve done. I broke his mother’s heart, but he still cares about me, and I can’t let that die.’
Anakin is very different in the end. The thing of it: The prophecy was right. Anakin was the Chosen One, and he does bring balance to the Force. He takes the ounce of good still left in him and destroys the Emperor out of compassion for his son."
-George Lucas
It’s supposed to be sudden and because of that it’s far more impactful to see him finally be selfless again after being a monster for so long. He goes out in a blaze of glory. It also makes it far more impactful for Luke to spare Vader despite the fact that he’s still evil.