Ive thought about this also. Mainly because after Vader stops Boba Fett from firing, and Han begins to calm Chewie, we see Leia look over at Vader, who simply stands and observes. Leia seems to sense that there must be a reason Vader did what he did. She then goes over to Han to say what could be her final words to him. Ive always wondered if Vader knew Leia was his daughter, or at least sensed there was something about her, and she should not be killed. Leia's importance to grand scheme of things, which would be revealed fully in JEDI, begins to show at that moment, followed shortly thereafter by her response to Luke's call for help, as he hangs underneath cloud city. Thats what I think anyway.