Well, I suppose the other way to avoid it is make all of those into +1 vorpal arrows. Instead of forcing you to roll a one, they just have to roll a double 20.
Yeah, it's extremely expensive. But they're affordable in a wish economy.
To avoid the whole "vorpal can only be applied to slashing weapons" thing: either use the modified arrow that does slashing damage, or argue they deal slashing damage when used in melee.
Since we're using abusable interpretations, sleight of hand + iaijutsu focus adds 9d6 damage to anything that hits (not that it matters, since you're probably immune to damage).
Oh, and apparently I'm on crack, since I believe the first round of concentration is covered by casting the spell. So double whatever silly number I had there before.