And then my browser crashed...
Story is Spuffy.
Pretty sure it's not on tth. I don't remember it being a crossover
It may be on the pit (fanfiction.net), but if not, the other two likely candidates would be here or Elysian Fields. Unlikely candidate would be AO3 or any of the other spuffy centric sites.
No idea of the name. no idea of the author.
I think Spike might have gotten them to a van in daylight and figured out he was semi immune to sunlight (when wearing amulet), but that may be a different story that I was part way through when the browser died... :-)
Thanks for your assistance!