Enigmas said:
Hey everyone, I've been following this thread after seeing Adywan's ANH:R and finding it to be superb.

I have to chime in here, though. When did Vader find out that "The young rebel who destroyed the Death Star" was his son? Vader says in the beginning of the film (upon seeing Hoth's shield generator) "That's it, and I'm sure young Skywalker is with them!" So Vader evidently knew then.

I've answered this funny enough after discussing this at length before:

A unknown Force-strong Rebel pilot ("The Force is strong with this one. . .") who destroyed the largest weapon in the Imperial arsenal undoubtedly warranted an investigation probably by Vader himself. Between ANH and ESB, Vader attacks Yavan and a Rebel pilot gets captured, and its from him Vader learns the name Luke Skywalker. Given that name and its connection Vader's past, he would have followed clues that would lead him back to his former home Tatooine and probably to his half-brother Owen. Along with the more than coincendental appearance of Obi-Wan protecting Luke, Vader could easily work backwards and make the connection between Luke's age and the events of RotS and realise that Luke was the child Padme was carrying (not knowing she was carrying twins). Its completely reasonable that Vader would know of his son by that point, even before the Emperor calls him on it in Empire Strikes Back.

I hope this helps.