Michael said he had skin condition that de-pigments his skin in the Oprah interview. He didn't use any technical terms. However, his doctors have talked of lupus and his autopsy reports have revealed about his skin patches. He himself had said it's in his family (his father's side- so it's genetic). Remember that Michael always used an umbrella when he went out in Sun. His blotchy skin is visible in his hands, neck and chest in those rare photographs where his skin was exposed (let's not forget that he always wore full sleeves and covered himself as much as he could.
With all these facts, there is no doubt that he had vitiligo and lupus. He has used make up and might also have used skin creams to even his complexion. He was in show biz and looks were very important for a visual artist like him (he was a dancer/dramatist on stage which is as important as his singing). Let's stop doubting him and I am glad that some awareness on a terrible disease is spread because of him.