Buffy is a potential, Spike can be vamp or human or another potential. Buffy can start out as a potential and then become a Slayer.

The ones I know are from coalitiongirl's fic rec list, so I'm scouting around for more. Or if you can second some of the ones from her rec list so I know which ones are best...

coalitiongirl. livejournal. com/ 69639.html

http://dark-solace.org/elysian/viewstory.php?sid=2123
Sideways by GoldenUsagi Rated: R
Summary:

Alternate reality. Buffy starts as a normal college girl. But her life is slowly turned upside down beginning with the day she's unexpectedly in danger. Hired by chance to protect her is Spike, a vampire with a reputation for doing business with anyone for enough money. She's less than thrilled, but finds herself drawn to him even as she realizes he's more than he seems. But humans aren't the only ones after her, and when she's called as the Slayer, her whole world changes. Things with Spike, who she's discovered some realities about, are no longer as simple as they seemed. But being sent to the Hellmouth and realizing the high death rate of her calling, Buffy hires Spike as backup with the only thing she has left—Slayer blood.

A Trunk Full of Trouble by madrog

http://dark-solace.org/elysian/viewstory.php?sid=1191

Summary: A car thief that doesn't believe in love is about to get stuck with a trunk full. This story is very AU, a world still with vampires and slayers, but does NOT follow BTVS canon. Buffy is a rebellious SIT, and Spike is his own man/vampire, not love's bitch.