But going back further, when that scene was written and filmed, Vader and Anakin probably weren’t the same person at all. Trying to think like Lucas makes my head hurt.

I don’t know. I sort of feel like he already had a feeling they might be one and the same.
Alec Guiness said that he insisted Lucas fill him in on background details so he could act his character better and I think he said he was told secret stuff.
Now note the whole meeting between Luke and Obi-wan. When Obi says Darth Vader killed his father, that whole bit, look at how Alec Guiness acts that, his eyes go all shifty, he is clearly stretching the truth (at best). He didn’t deliver the line and act it out as if that was simple flat fact.