Maybe W&H were just bluffing. Remember, this is Harmony we're talking about, and she's not exactly the sharpest knife in the drawer.
On the other hand, maybe the human soul of each vampire actually is "just floating around" somewhere, being held in some kind of stasis, unable to go on to whatever reward or punishment that their mortal life had earned for them until the vampire is dusted. That would, at least, explain why Angel and Spike were able to have their souls restored without the "being ripped out of heaven (or hell) trauma that Buffy experienced when she returned ib Season 6.