Evil Angelus 95
Total Dork
That might be true. It would depend on just how specific the spell actually is, which, of course, we don't know. Obviously, it isn't all that common a thing to do, or there would have been other vampires cursed with souls, which, apparently, there never were. Still, with Angel's spell as an example, it should be easier to tailor one to fit this situation.
Then again, this is a totally different situation, since it would involve installing the soul of one person (human Willow) into the body of a totally different person (Vamp Willow). Angel's spell is totally different.
Maybe they could somehow get to Pandora and convince a Na'vi shaman to hook Willow and Vamp Willow up to the Tree Of Souls and do the transfer that way.
(If I mentioned Ras Thavis and a brain transfer, would anyone here know who or what I was talking about?)
Would it be that hard? I mean, Vamp!Willow is basically the same person as normal Willow, it's just that she's from a different realm, and she's a vampire. Other than that, she was, for all we know, the same exact Willow we saw in "Welcome To The Hellmouth"