I think I read somewhere that in the original ROTJ novelization (which I believe was supposed to be canon, might be wrong and not sure if it still is even if it was), Vader does disappear and Luke does only burn his suit.
That wouldn’t explain anything because we see Vader die on-screen and he doesn’t disappear immediately, unlike Obi-Wan and Yoda.
Obi-Wan disappeared more immediately than Yoda, who kind of faded away slowly after he took his last breath whereas Obi-Wan was gone instantly before the lightsaber even went all the way through him. So that’s already inconsistent. You can come up with some sort of convoluted Lucas logic to explain why Vader might take even longer, like the residual functioning of the suit kept his heart pumping or the Force had to take extra time to sort out his soul because he was Vader and the chosen one or whatever bullshit. We don’t actually see his corpse for long. It’s plausible that he disappears right after the scene cuts. Point is it makes at least a little more sense than him just not disappearing at all.
I guess the other explanation could be that something about the cremation ritual would allow both Vader and Qui-Gon to become force ghosts, like only if the body is gone can you be a ghost. If that is the case, then perhaps that is the “secret” Qui-Gon discovered and passed on to Obi-Wan and Yoda, who then used the Force in their last moments to will their bodies to disappear so they could live on as ghosts. (Which is consistent with the meditation stance Obi-Wan appears to take right before he dies, like he is putting all of his remaining energy into something.) But it wasn’t necessary for Vader since he was cremated anyway.
That only really works though if the Jedi did not routinely use cremation and Qui-Gon was an exception for some reason. If they did, you would think there would be lots of other dead Jedi ghosts who would have communicated with the living at some point. But I don’t know what else makes Qui-Gon so special, and is also consistent with Vader becoming a force ghost despite not disappearing on-screen. The fact that both were cremated is all they have in common, and the thing that all the known force ghosts have in common is that their bodies dissolved somehow, either by incineration or evaporation.