Anya and Xander are in a relationship. Spike is in love/like (I think it's love) with Xander. Anya goes to Vegas for the weekend and Spike stays with Xander to keep him company (they're friends) and while snooping through Anya's stuff he finds a letter that proves Anya's cheating on Xander.
That's all I can remember. Virtual hugs to anyone who can help. This has been bugging me for three days now.