I think the difference is that Vader was redeemed as a father and not necessarily as a man. If he didn’t have the good fortune of dying when he did, he would have found himself dangling from a rope at Nuremberg, so to speak. I do think that OT was too easy on Anakin, but he died before he would have had to face any consequences.
But who is left to forgive Ben Solo? Luke and Han are dead. Probably Leia, too. Rey doesn’t owe the little shit lord anything.