I strongly disagree. Boromir was not completely evil. He was tempted by and desired the power of the ring. But he was at war within himself. True, Vader eventually struggled in his conscience when he met Luke, but for a good while he was "totally" bad. Boromir was never that far gone...
Good point. Darth Vader certainly seems to have shut the door on any possibility of redemption, until the very end. The Smeagol/Gollum comparison is more valid. And probably, this points to Tolkien's vastly deeper understanding of human nature than Lucas's. Tolkien seems to understand that when you're too far gone, the chance of you returning to the Light is incredibly slim. In the end Gollum triumphs over Smeagol. I can't say without seeing the final Star Wars movie, and Anakin's final turning, but the return of Vader to the Light on witnessing the suffering of his son is probably too easy. The only sensible explanation is that the conflict was there all along, and that the suffering of Luke was just the final straw that brought him back and not the complete picture.