The only way I see a plotline of Vader trying to destroy the Empire from within working is if he’s doing it for selfish Sithy reasons, wanting to take Palpatine’s place and continue the whole “rule the galaxy as father and son” thing. Vader is not the kind of person to admit that he’s wrong, he’s the type to double down based on his emotions. Because he can’t let go of anything. Through the whole movie he keeps avoiding the truth and refusing to admit to his feelings. “If that is your destiny.” “There is no conflict.” “If you do not fight, then you will meet your destiny.”
Stretch that out to cover a whole movie instead of one scene, with Luke questioning him more and more. Vader’s realization in the middle of an ROTJ equivalent is that “I did the right thing for the wrong reasons, maybe Luke has a point,” after Luke voluntarily gets captured and stresses the good in Vader. He would have changed for the better, but still think it’s too late for himself to fundamentally change. That is until he’s presented with Palpatine killing his son, when he realizes that he can truly do something good for the right reasons (saving his son and killing his master directly).
I hate to bring up another comparison, but Frieza/Cell Saga Vegeta is a close example of this kind of arc. He may be trying to help the heroes, but he’s still doing so out of awful reasons/despite the badgering of his allies. The first time he does the right thing for the right reasons is to shoot at Cell so that Gohan can overpower him. Like Vader, he does so because this man tried to kill his son (well, Cell actually succeeded for a few minutes).
That kind of arc works and produces a satisfying redemption. However, it sadly wasn’t what Lucas wanted to tell and the saga’s story suffers as a result. But hey, I buy Vader’s redemption infinitely more than I buy something like Snape’s redemption.