*disclaimer* Keep in mind with my reply that I did not read the novel (if there is one) of ESB, and it's been a while since I have seen the movie. An interesting thought I had some time ago while watching the movie was that while Vader was looking for Luke on Hoth, he found Leia. I think he was picking on on some of her "force presence" if you will while she was in a stressed and anxious state of mine. He may not have thought about it at the time, because he knew Luke was nearby. However, in Bespin, with Han about to lowered into the Carbon Freeze chamber and the scuffle started to erupt, he picked up on it again and kept Boba Fett from shooting her. After this, he altered his deal with Lando, because in his mind, he knew something was up. It wasn't until ROTJ that he put it all together.