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.
Lucas may have already thought of the idea but didn’t tell her or wasn’t certain he wanted to go through with it yet.
The fact is that we’ll never know for certain exactly when Lucas came up with the idea. There is a lot of conflicting information about how much of the story was planned from the start and how much was improvised.