Well, I disagree, I think it did disappear, and that his ghostly appearance at the end proves it (because I am certain that disappearing and ghost form are linked); in support of all this are comments by Rick McCallum and the Official website (though that is not proof in of itself).
In which case, I wouild say it disappeared very shortly after his last words; probably by the time Luke took off in the shuttle.
Wel, we saw he was dead but them Luke took him in his shutlle so we don't know what is the full truth, but everybody and GL are saying that it was his armor that was on the pile, this was because GL first wanted to get the disapearing trick only done by guys from the Ligh Side, but felt it was not fair for DV so he let him be a ghost also.
It is true that it is ANNAKIN that is a ghost and NOT DV!!! There IS proof in the movie ROTJ!! Look at the gost his hair! He HAS hear so he is not Vader without his armor wich we saw earlyer in the movie, so this MUST be Annakin
I don't think of it as a trick. I think of it as more of a "soul", if you will. Anakin/Vader have just one soul. Since Anakin was prepared to die, he COULD become one with the force, which I think Qui-Gon couldn't b/c he wasn't ready. Sorry if I am using too many religious metaphors. Maybe you don't have too disappear to enter a "spirit body?" If true, Anakin wouldn't have had to disappear for Luke to see his "spirit body."