I have a suggestion to start off. There's a much easier way to avoid the whole problem of Obi-Wan lying. It's not quite the route that was already discussed, but I think it could work well.
Simply omit the reference to Vader killing Anakin. Remove the question that Luke asks -- "How did my father die?" Rearrange other dialogue as necessary to make things flow.
I'm quite happy about how this entire scene plays out as is, myself (which is what the BigMigMaker in my project is all about, of course), but I believe the best way to remove the whole problem and avoid losing the impact of Vader's revelation is to simply make it so Obi-Wan never tells him anythign concrete about his death. focus on Obi-Wan talking about Anakin's life. Make it seem as though Obi-Wan wants solely to concentrate on the Jedi, on Luke's father's lightsaber, not the dark times of the fall of the Jedi. Mention that Anakin and Obi-Wan were close friends, somehow including the "He was a pupil of mine" exchange. But avoid the line about Vader killing Anakin.
I believe it can be done, but I don't have much time to write it up this morning after work. Perhaps someone else could take a stab at the idea to see if it plays out well enough?