Gonzo has an interesting point over here: http://blogs.starwars.com/gonzojedijournalist/2

But I'd like to reply with an alternative theory, or at least a supplemental one. First, I've read that Vader has tango'd with Han in the past (prior to ANH), and that Vader was torturing Han for his own enjoyment. I don't think he tortures Leia at all, at least not that we see. And as for the noise that was torturing Chewie, I believed that was just to "stun" Chewie so they could bring people in and out of the cell without a wookie ripping them limb-from-limb.

All of this said, Vader could have also done it to lure Luke in via the force. I wonder if this was so, did Yoda know what Vader was doing? Did Yoda fear Luke would join the darkside? If so, did Yoda finally wake up and know what evil was trying to do as he was previously blind with Anakin?

I'm not sure, but I'm reminded by something from the Matrix believe it or not when the Oracle says we can't see past the choices we haven't don't understand. Perhaps Yoda did not understand enough about father-son relationships to see what Luke would do when confronted by Vader with the truth, or even if Vader would expose the truth.

PS I just watched ESB, and in the scene where Vader tempts Luke, you can see the top part of his helmet moving a little in the Wind. I'm suprised they let that sneak in eh?