DF Shadow said:
Well another reason GL changed up the Vader/Palps dialogue a bit and added "How is that possible?" was to ackknowledge the fact that the Emperor told him he'd killed Padme.
So it was a PT retcon continuity patch.
Right, but in the opening crawl it stated that Vader was 'relentlessly searching for Luke' or something to that effect, and when the probe droid image of Echo Base is looked at by Vader, he says "That's it, the rebels are there and I'm sure Skywalker is with them."
So when Vader talks with the Emperor, he already knew the the name of the person he was following. As if the name weren't enough, Vader says "Obi-Wan can no longer help [Luke]" in that same hologram scene in the asteroid field. So Vader already knew Luke had been getting help from Kenobi. Vader would have to be incredibly dense not to have resolved the clues by then.
-Force is strong with this rebel
-this rebel was associated with the Jedi Master Obi-Wan (Vader's old master!)
-And he share's Vader's last name of Skywalker
I think it was made fairly clear that he had already figured out Luke was his son. So in my mind, the prequels just fucked up, and the retconed hologram scene simply capitalized on the mistake. Similar to what happened with Leia remembering her mother on Endor, except no retcon was even attempted.