Well, hell, even in the continuity set up by the prequels, Vader telling Luke "Obi-Wan killed your father" could have made perfect sense. Like someone said, Obi-Wan did chop off Anakin's limbs and sadistically left him to burn to death (to borrow from Confused Matthew), and Anakin was under the impression that Obi-Wan was banging his wife, so, from a certain point of view, I could see Vader trying to make Luke swallow the pill that Obi-Wan was evil. And then in the next movie, Luke would have had to rationalize his loyalties, and Obi-Wan would have to prove why exactly what he did had to be done. Hell, just from the context of the original trilogy, you could have both revelations. You could have Vader tell Luke Obi-Wan tried to kill his father, then have Obi-Wan admit to Luke that that was indeed the case, but that he had a very good reason for dimembering Luke's lifelong hero: that he is, in fact, Darth Vader, but Obi-Wan had been too afraid for Luke's feelings to tell him.