I absolutely love this one! Dawn does not belong to Buffy, but she and Spike have to work together to first find Dawn, then protect her in order to avert the apocalypse. It should fit your criteria!
Dark Prophecy by
PipergirlSummary: Buffy and Spike must put aside their differences to find a baby whose sacrifice will bring about an apocalypse (yep--another one). This will be an adventure that will test their mettle and help them learn about each other and--most importantly--about themselves. A Slayer who's afraid of flying, a vampire who's afraid of... well, he just won't say, and a few really colourful characters will hopefully keep you entertained.