Hang on. Vader DID disappear at death. Otherwise he wouldn't have been in ghost form at the end of ROTJ, which also knocks the surprise theory on the head, I'm afraid.
As for some form of new evolution, that is entirely possible. The whole disappearing trick is meant to be an Obi-Wan/Yoda plot for the next two episodes anyway, so it will be something mysterious.
Here is the simple list of facts that we know:
1. Obi-Wan did it, at a point when he wasn't even wounded so it can be done at will.
2. Yoda did it, when he was critically ill.
3. Vader did not know it was possible (which annihilates the Sith-trick theory).
4. Yet Anakin was still able to do it when he died, presumbaly making him the third ever Jedi to do it (unless someone like Windu does it first).
5. Qui-Gon does NOT do it even though he had plenty of time, nor did anyone expect him to
Oh and Queeg, seeing Qui-Gon was dying at the time, I think getting Obi-Wan to do the job for him was the best solution available at the time. If he had had eight hours dying time to impart some more vital wisdom on Obi-Wan I'm sure he would have done, but needs must.
And Yoda's decision to NOT train Anakin may be HIS big mistake, so Yoda may be in the wrong there, and Qui-Gon in the right.
Besides, ultimately, Anakin came good in the end. So Qui-Gon WAS right, and there's really no other way to see it.