C3PX said:
I always thought Luke was using the force to block Vader out. Notice Luke is concentrating really hard. Vader finally picks up on his thoughts about his sister, I always assumed that was because Luke wasn't strong enough in the force and Vader was able to see through his fog just enough to pick up that little tidbit, likely because Luke was worried about Leia at the moment and thinking too much about her and what it would be like for her if he failed.

Yeah, this is how I've always intrepreted it as well. (I wasn't aware of any of the EU "explanations"). Obviously, Luke is panicking and he happens to think of Leia, and he's mortified once Vader picks up on it.