The question is, how to involve Vexen in a way that ISN'T going to make him want to tear his own face off.

If Roxas is the 'bride', he could probably stand to be 'bridesmaid' as long as a) no one tells him that's what he actually is (I mean, it's two guys marrying, so they're both grooms, rly) b) Roxas asks, c) NO DRESSES ARE INVOLVED.

Of course, he can always be the obligatory party pooper who sits out. XD