Dim, that's because the relation between a ghost and disappearing aren't fully proven. But you can say that OB1 disappeared and he became a ghost, same with yoda. But you don't see Vader disappearing and that's the problem.
Ush and I agreed that Vader DID disappear (for example; watch the burning at the end of ROTJ, the mask of Vader is almost lieing in his helmet, indicating no-one is in the suit+ everybody at sw.com and lucasfilm is saying it, but that's not the same)
BTW: his costume isn't natural or handcrafted, it's a difficult high-tech suit, so it didn't dissappear BUT Annakin (not Vader) DID appear with his clothes on at the end of ROTJ.