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.
I do have to say I like the way he "balanced the Force" - two Sith, two Jedi, nobody else touches the stuff. Prophecies are always dangerous to interpret...
It is my understanding that George Lucas was very much influenced by Joseph Campbell's "The Power of Myth" when writing Star Wars. It may be the difference between Tolkien's Catholic worldview, and Campbell's pagan (in my understanding) worldview which colors the way the characters are created and grow.