I read this fic a while ago and have been looking and can't find it anywhere. In it Buffy's a vampire, and Spike's the slayer. I'm fairly sure they call Buffy, Elizabeth the Bloody, though I may be mixing it up with others. In it she's a good vamp, maybe has soul though not sure, I remember something about her not being violent until the Watcher's Council killed her family, and after that she was violent, and she killed the Watcher's responsible. I think her family was a mother and little sister like in the series. I don't think she was a slayer before she was turned in this fic, though I'm not completely sure.

Mmmh diifficult. The first name I though of was Maryperks though. More Buffy!Vamp fics here.

If it is Maryperk's story - I'm not sure about the part where her family got killed by the Council, can't remember that, then it is:

Save My Soul by maryperk73703 or here
William Jamison Giles is the Slayer and Buffy is a vampire with a death wish. When they meet the sparks fly and things get really interesting in Sunnydale. Lots of familiar faces in this ficcy, just not used in the same way as canon.

I still haven't come around to make a "Spike the slayer post" but I know here is a threat. Maybe you'll find it there.
Link to fic rec forum entry - my Spike the slayer list is posted there.

Unfortunately Save My Soul isn't it. And I can't seem to find it on the others you suggested. But thanks anyway for the links, all those vampBuffy fics are great. there were I think a number of similarities with Save My Soul, with the whole spike being the slayer, buffy being a vamp and I think just arriving in sunnydale around the start of the fic. She was also vampiricly related to Dru, Angelus and Darla as well, she basically I think took over spike's role before the series.

To go to the Forum entry click here, this is the other place I mentioned where you can find "Good vamp Buffy" fic recs.
My links for Vamp-Buffy! fics is *click* here

Hope you'll find it or someone knows. (I'm out of ideas what it could be.)