Well, I figured that Vader knew that Obi-Wan was going to die, but not that he would vanish, much less ascend.
In the context of the PT, this makes even more sense. The Sith were obsessed with material immortality, rather than spiritual immortality. Apparently, the possibility of spiritual immortality was unknown to either the Sith or the Jedi, until Qui-Gon secretly contacted Yoda.
That in and of itself is interesting. Luke starts off knowing only what little Ben showed him. But by the end of ANH, Luke's in on a big and profound secret—spiritual immortality—unknown to the Sith or the Jedi Order. That's probably why Obi-Wan refused to help Luke fight Vader, to keep that secret from the Sith.