I am definitely going to refresh my memory tonight by watching ROTJ... but do we actually see Vader / Anakin take his last breaths when he is lying there with Luke? I seem to recall Luke and Anakin talking about Leia... blah blah blah... then a few moments later we see Luke trying to get Vader into the shuttle.
I'm not sure I agree with some of the theories I have read here. I'm not saying they are wrong because who am I to say... But anyway --- I don't think it is about being ready for death. Something just tells me that that would be too easy to figure out.
It has to have something to do with acceptance. Meaning that Luke "forgave" him for turning to the Dark Side and thus accepted him back as a father. Being as Star Wars has religious implications... maybe it has something to do with salvation. I don't want to get into a religious discussion... but anything is a possibility.
But if you think about it... he could have disappeared and all that was left was the suit. Who knows. I think GL would have made it clear if Anakin disappeared. Anyone have GL's number? I will call him and ask him!!!