At the end of three, in what is almost a throw off line, someone orders 3PO's memory be erased.
In ANH, I think Ben was attempting to maintain the subterfuge until he saw the message from Leia, at which point he tells Luke about Vader "from a certain point of view" and gives him the light saber.
Oh, and I share your sentiments about the poor acting and script. It's really too bad Lucas tried to do so much with episodes I, II, and III. He needed Kasdan helping write the scripts, and someone around him to tell him "no" on occasion. It's like we all can see where he was going with the stories, and I actually like that direction, but the bad acting and script just overwhelm that part of it to the point of it being a huge mess.