When did Vader find out about his son?

This question has kind of bugged me. In the 3 years between ANH and ESB, at some point Vader had to find out that Luke was his son. How do you think he found out (word of mouth most likely) and what would his reaction be. I see it as being almost as shocking for Vader as it was for Luke to find out he was his father.

That would have been awesome to see. Here his is, alone for 19 year with the death of his wife and unborn baby, and now he finds out that his child survived. Imagine the drive to find Luke and meet him for the first time. I bet he was thinking, "wow, he has my looks..and his mother's height."


TLJ Edit: Please see the index before posting a thread. You would have found this thread there.