Me and Cibernite are divided in that respect.

I don't think the side he takes (Rebels or Imperials) should be ambiguous. Don't get me wrong, I love the hell out of morally dark gray characters, but I think it would be undermining any sort of redemption for Vader if applied in this case.

How do you atone for hacking up toddlers? Spend the remainder of your diseased life wholly devoted to defending the weak and helpless. Save several times more children than you murdered.

Does that truly atone for the actions? You could argue yes or no. But the point is, Vader would be deeply obsessed with correcting his past mistakes. As long as in his own mind his evil deeds are never truly atoned for, then the good he will do will end up greatly outweighing what he once did.

Part of the atonement is that he'll never forgive himself. A self-punishment comparable to a priest breaking out a whip and flagellating himself non-stop.