Fist... Vader did
become one with the force, Luke is only burning his suit... you can see Anakin's ghost with Obi's and Yoda'...
Now, the reason why Qui-Gon, like all the jedi we see die in ep2, didn't dissapear and become one with the force... Well, there's only theories at this point, we might lean the reason in episode 3, then again we might not. So your guess is as good as mine... well maybe not
It's true Yoda trains all the younglings... and that's a fair answer. But the true reason is at the time the OT was released, Qui-Gon Jinn did not exist in George Lucas' mind yet... Kinda boring explanation but it's true... Still, like I said the yoda training all the younglings explanation holds up pretty good to me.
Well, we had a very interesting discussion
about it... and we finally got to the conclusion that it all comes up to wether or not you consider EU as part of the true Star Wars story
or not... personally I don't...
--> Next time, don't make 2 identical threads in different forums please