Somewhere in your theme probably. vBulletin may not support PHP in themes, so it’s possible you may need to use some other method to include the file. I don’t know anything about vBulletin themes though so don’t know for sure.
It will work fine with vBulletin, I’ve just never used it to know how vBulletin handles file includes. The plugin will work fine with pretty much any half decent software out there, only very crude or poorly developed softwares won’t work with it.