"If he could be turned, he would be a powerful ally."
I had a thought about this line the other day. Now, it's obvious that Vader is trying to spare his son's life here. However, I had another thought. See, knowing what we now know from the PT, there are only ever two Sith. Is it possible that Vader was offering his own life to the Emperor in exchange for his son's?
The more I think about it, the more it makes some sense. Even though Vader does offer Luke the chance to join him and destroy the Emperor, he's urging Luke to kill him throughout both of the duels. "Only your hatred can destroy me," for example.