Alrighty. Been kicking around a coupla ideas, both hinge on whether or not Kurin really has Perigeron's soul.
If Kurin does have Perigeron's soul, simply killing the vampire may not free the soul. It will most likely have to be wrested from the vampire. Could Alethia's demon acomplish this? I was considering him snatching the soul while Alethia casts Crown of Flame, searing the vampire with holy light. Another spell alternative would be, if Matt allows...
Phoenix fire page 103 BOED. Alethia would be willing to cast this spell. However, it is a seventh level spell.
The only way I can think of this working, is if she had a special, wondrous item made for this spell. That would be a 7th level spell x a 13th level caster (minimum) x 50 + the item the spell is going to be imbued in, in this case a phoneix feather.
So 7 x 13 x 50 + 50 for the phoniex feather would be 5,000gp. Perhaps Aleena could have it made quickly... Or one of the churches could have one in storage. It's not something done lightly, but it is completely in keeping with Alethia's character, as well as in keeping with the demon's character.