And speaking of this scene, as well as Return of the Jedi in general, I think Luke being vulnerable for the briefest moment and allowing Vader to read his mind and learn the truth about Leia actually serves a really great purpose.
To me, this really is the biggest reason why Obi-Wan and Yoda had to lie to Luke about the truth about his father, and why he wasn’t “ready” for it. Yes, I think there is a simple answer, Luke wasn’t emotional mature enough, but also imagine if Luke had told Vader from the beginning and then Luke had an encounter with Vader. That would be all Luke would be thinking about, and a young, novice Luke would practically be holding a sign above his head reading “I’m your son!”

Luke needed to be prepared to “bury his feelings deep down”, and I think they would have eventually told him if things had gone differently. I just don’t think they ever expected Luke to become a hero so quickly as to have Vader try to find out who he was. In a different scenario, I think they would have tried to train Luke in hiding until he was ready to face him, but the Force obviously had other plans.