This have been driving me nuts!
What I remember is Xander and Anya are together and they go on a holiday to Vegas and Spike house sits only to find out that Anya is having an affiar with some guy. He is really upset and then Xander comes home early as who has also found out and then the boys get together. I remember that the boys do a calling each other thing and are really good friends and Xander has the house he has done up which is why he asks Spike to House Sit while they are away. Can't remember anything else and it is driving me insane!
*ETA - FOUND - Back with Me by Alexandria Brown link in the comments if anyone else is intrested