I don't think there's much official information regarding 'saving' someone's soul, particularly after they're dead. My personal take on the undead/no raising thing breaks down something like so:
Undead do have souls, after a fashion, but unlike the case of a normal, living mortal, the soul and the body are not co-existing seperate entities. When an undead is created, the soul of that person is bound into their flesh, using it to reanimate the corpse. This is why you can't raise/res a currently active undead - the soul is otherwise occupied already, as part of the undead creature. Likewise, this is why you can't raise a destoyed undead; the process that binds the soul to the corpse when the creature is raised makes them inseperable. Destroying the body also destroys the soul. More powerful magics such as res and true res, which can reintegrate someone from just a fragment, however, can reassemble the once-living person, mending their shattered soul just as well as their shattered body.
Now all that just touches on the ability recover the vampire's soul after the creature has been destroyed. As for saving it or is it corrupted forever, well, look at it like this...
...even if you brought 'em back to life, it doesn't necessarily change their alignment. Evil is as evil does.