The Leigh Brackett draft of TESB didn’t have Vader as Luke’s father. That’s a strong enough indicator to me that Lucas hadn’t thought the idea up yet.
Well that draft was obviously written post STAR WARS so it’s possible he could have strict orders not to include that bit.
Also this is pure conjecture but perhaps the “No, I am your father” plot twist was a throw away idea that he had while writing STAR WARS and revisited the idea while making ESB.
EDIT:
Density beat me to it dammit