Although I've never been a guest at a Halloween wedding, we did cater one, and my mother directed it.
It was really a lot of fun. The groom was dressed as the Phantom of the opera, and the bride as Christine. And all the guests came dressed in cotumes.
It was not "Satanic" or anything, it was just fun. The vows they took were standard Christian vows, though the ceremony was not done in a church.
Just have fun and treat it like any other costume party.