Would you not say Vader finding out that he has a child is a BIG revelation in his life? And even more he realise even more so how the Emperor has been lying to him. That revelation is the driving force of what he's doing through out ESB. He's eager to find Luke before the Emperor can get to him. When the Emperor talks to Vader about the pilot that destroyed the death star it was like the cat was out of the hat. Now Vader had to lie to the Emperor.

Anyways we can go back and forth on this. (If i have time i'd love to make a quick cheap version of this scene using my action figures. Maybe after i finish my Back to the Future Dub movie. Almost done with that.)