Disappearing doesn't necessarily imply that one will return as a ghost. In the films, we've got at least one dead Jedi who doesn't disappear but does have some ghostly powers, and one dead darksider who blows up but doesn't have any ghostly powers. (He gets them in the EU, though.) Vader was probably familiar with the idea of Jedi disappearing at the moment of death; even if none of the Coruscanti Jedi did, he had probably seen other proscripted Force users die in such a manner, or read about it in his studies.

Even if Vader was aware that disappeared Jedi always reappeared as ghosts, he probably wouldn't have understood the "more powerful than you can possibly imagine" line. Ghosts aren't inherently powerful. What made ghost Obi-Wan powerful was the fact that he had bonded with Luke, a potential Jedi and the Son of the Suns. Had Vader known that Obi-wan's ghost would be coaching Luke at Yavin and giving him directions to Yoda's planet, he probably would've taken him alive.

Obi-wan's power was not inherent in his disappearance or his reappearance, but in who he knew and what he could maneuver that person to do.