So how come when Yoda and Obi Wan die there bodys disappear but when Vader and Qui-Gon die there bodys need to be burned?
Because… STARRRR WARSSSS
But really, I’m not entirely sure myself other than, it’s whatever the writers wanted to do.
Maybe Qui-Gon is the first Jedi to die on film, so they burn him. Then Vader is the last ‘Jedi’ to die in Return of the Jedi and is burned. It fulfills Lucas’s “poetry it rhymes”
Only botched up explanation I have.