On the one hand, Yoda, Obi-wan, Vader and the Emperor are the only Force-sensitive deaths in the original trilogy (which I know some people use as their only canon). Vader's funeral pyre could have been just the electronics/prosthetics left behind.
On the other, Qui-gon dies in Ep one and leaves a body for a pyre. I don't recall the massacre of the Jedi being on screen (seen Ep 3 once, in theaters), but I would think if all Jedi poof when they die, some of them could/would have been shown as disappearing. Vader's reaction to Obi-wan's death struck me as confusion - rooting around in the robes as if looking for something. Not a reaction I would have expected given the number of Jedi he has killed in the past if all Jedi disappear. My conclusion to resolve these facts was that it was something deliberate Obi-wan and Yoda did to disappear, even if prep time was very short. Although this doesn't explain the Anakin force ghost at the end. I admit it is also possible the early books of the extended universe could have influenced me as well.
On the other, Qui-gon dies in Ep one and leaves a body for a pyre. I don't recall the massacre of the Jedi being on screen (seen Ep 3 once, in theaters), but I would think if all Jedi poof when they die, some of them could/would have been shown as disappearing. Vader's reaction to Obi-wan's death struck me as confusion - rooting around in the robes as if looking for something. Not a reaction I would have expected given the number of Jedi he has killed in the past if all Jedi disappear. My conclusion to resolve these facts was that it was something deliberate Obi-wan and Yoda did to disappear, even if prep time was very short. Although this doesn't explain the Anakin force ghost at the end. I admit it is also possible the early books of the extended universe could have influenced me as well.