He doesn't say he always planned this to be about Darth Vader since 77, you all just keep misunderstanding him when he says that even though he -didn't- always plan it to be about Vader, that he slowly realized that it alwas was as he went along.
I think it's kind of funny to watch people who don't like Lucas protect the world from the "distortion of history", to "protect" future generations from Lucas' so called "revisionist history" when nobody is distorting things more than these same people.