Well I think it was at least before post production, I remember an interview with Mark where he said George pulled him aside before shooting the scene and told him so he would know to react stronger than expected.
To be more clear, I was proposing that the idea of Vader being the same person as Annikin/Anakin - the man the Ben talked about in the first film - was not put in place until Lucas started on ROTJ, or very late in (post) production for TESB (my more ‘charitable’ estimate). Vader was indeed Luke’s father when TESB was made, but he was someone else under the mask - not Annikin/Anakin, but possibly another Skywalker (brother of Annikin), or even Ben’s son. Even if the relationship wasn’t a paternal one, still some sort of familial relationship and ‘secret identity’ was meant to be there when the first film was being made. However, the idea was temporarily set aside in order that the movie would work as a stand-alone.