Sort of:
Star Wars Spuffy Style by moxie
Summary: Princess Buffonia meets rogue Vampirian smuggler Spike while running away from marriage to Riley Sleepwalker and a strange man who keeps harping on about her destiny as the Slayer. Not a cross-over. More a melt-across. And not to be taken in the slightest bit seriously. this is my little break from all the angst. All Spaceballs, cliches and badfic references are entirely intentional...
The Truce by Eternal_red
Summary: Once in a galaxy, far, far...oh, that's been done. Right then. Humankind has been at war with the Aurelians for decades. When the two sides finally reach the point where negotiation is better than annihilation, a truce is called and a treaty negotiated on the planet currently under dispute. The peace treaty is cemented symbolically by the joining in marriage of the two youngest soldiers present, thereby throwing Buffy and Spike into a relationship that they really, really, don't want. Although not a traditional Vampire versus Slayer scenario, it is close enough to be familiar.
There is a story where Spike is escorting a princess (Buffy) to her wedding in another fantasy kingdom but I can't recall who wrote it. I haven't read it yet so I don't know if there are beasties. | 
