My initial thought after my first (and only, so far) viewing of RotS, was, "Ooo, yet another Lucas continuity error!"

Then, after that, I had had the vague-ish notion that becoming a Force Ghost post-death was what the Emperor had been alluding to about the Dark Side trick of becoming immortal. That he was mistaken about it being a 'Dark' skill and that Vader had gone on to research and ultimately master it. Unlike Obi-Wan, however, he was unable to just *poof* into the Force upon his immediate death (leaving no body). And yeah, perhaps Obi-Wan and Qui-Gon were standing by to jump Anakin into the Ghost club.

Then, I just shook my head and said, "Whatever!" LOL sorry, I'm not any help on this!