I have a list of fics where Buffy is a vampire. It's
here. But I don't remember the fic you're looking for at the moment, not sure if it is there.
Good luck finding it, maybe someone knows it.
*edit* There is one (forgot if Faith is a friend of Angel) - His Death Wish by xoChantelly WIP
Buffy is a 200 year old vampire, turned by Angelus. He turned her in the nastiest way and now that he has a soul, she wants to get back at him. But wait... What happens when she falls in love with a human who happens to be a criminal with a death wish different from the one she has planned for him? And what is she going to do about the Slayer, Faith? EvilVamp!Buffy fic intended to be dark and real angsty
https://www.spikeluver.com/SpuffyRealm/viewstory.php?sid=34671But! This one is taken down; quote the author:
I just wanted to let you all know, well for those who read them, that I will be posting them again, but they will be revised and hopefully better lol.