Keep in mind this is only another theory. Maybe yoda knew it and wanted it to be only used for emergencies, so when the Jedi where all but exterminated he taught this to obi-wan just in case something was to happen to him and he needed to do something else of great importance. And Darth Vader probably figured it out how to do it in Ep V.
A very interesting mystery we seem to have on our hands...I always wondered about Yoda and Obi-Wan vanishing upon their deaths. Yet allegedly the reconverted Anakin did the same,leaving only the Vader armour behind. However, if one checks out an obscure passage in "Champions of the Force"(the final book in the Jedi academy trilogy by Kevin Anderson), Luke Skywalker wondered why his body did not vanish when he thought he was dead,following the betrayal of one of his students at the academy. He thought it meant he was not a true Jedi Master. However,as we all know,he wasn't really dead,it was only a trick by the Sith Lord in the book. So whatever the reason behind the disappearing,Luke did not know how to do it.Which only serves to enhance this liitle mystery of ours...
As a child growing up on with the original Star Wars(aka A New Hope), I was always fascinated with Obi-Wan's famous disappearing trick seconds before Vader's lightsaber blade cut him in half. I never understood why or how he just vanished like that,unless it was so he could continue to direct Luke in the ways of the Force,and eventually send him to Dagobah,as in ESB.
You Fools if you watched the third movie(6th one made) it says at the end that quigon-jin figured out a way of revival so maybe obi-wan made contact with yoda and darth sidios showed darth vader how to do this as well explaining all your problems. And they could just bind their spirit through the force
If people even watch ANH anymore, Obi-Wan disappears before Vader connects with him, because his cloak doesn't get cut in half. And about the disappearing trick, I agree that only Jedi ready for death do it, and we don't hear about it in the past because it has no relavence to the storyline. And plus, any Jedi can learn the pathway to immortality and ghost form, I.E. Qui-Gon didn't fade away, but he figured it out, and taught the others. That's just my two cents.
HEIL BARON VON RICHTHOFEN! HEIL THE LUFTWAFFE!
Do you think it's possible that, as Qui-Gon did with Yoda, Yoda or Ben or even Qui-Gon came to Vader and taught him how? Or that it was just simply that he was the Chosen One and did not need to learn it?