1. The Vader helmet's sight is limited.

2. I like the idea of Luke "masking" himself with the Force-never thought of that one.

3. It's the tradition of the camera frame defining what is in the character's conciousness. The Good, The Bad, and the Ugly 's end scene takes place in a graveyard surrounded by vast mountain vistas for miles around. Yet somehow Angel Eyes comes up out of nowhere with a shovel to surprise Blondie and Tuco. In the camera frame!

I always did wonder about this scene though. The set is so small that it is kinda obvious where Luke would be, but the "masking" effect gives it some credence-this may be why Vader was able to pick out Leia from Luke's mind as he was concentrating on not being found.