As I recall, Valen chose to become a vampire. Correct me if I'm wrong of course. Not only that, but she was chosen because of her lack in her previous life and her potential for being a killer.Heres my take on this subject....
one....did valen to be a vamp in the first place and if she didnt then she isnt inherently evil therefore even with the loss of her soul and killing people she still has the memories of feelings and love and compassion.
two .... a great way to implimit a love quest for valen is to stop her from killing over time even if you do kill innocent people in the beginning ....a redemption mod that somehow involves valen bringing down her master, bohdi, or maybe involving taking down the entire vampire circle.
three....the bottom line is this if a person is good there job is to cleanse the world of evil whether its destroying it ,converting it or "loving it to good"....so it could be done .....after all if anyone ever watched the series "buffy the vampire slayer" then they would know that vampires can be powerful forces of good.
just my thoughts
Let evil be evil, and good be good. There are plenty of 'good' mods already out there, and even more in the making.