Since Admin isn't chiming in here, is the vBulletin forum post that explains PMs are truly private.
http://www.vbulletin.com/forum/showthread.php?t=146965
If admin knows how to read the DB it sure would be allot of work to find one PM in a DB that is huge. I have tried it before, but it's not so easy. The method I explained would work best.
Again nothing will work if the PM was deleted. A better solution would be to find out from the person who was offended who the culprit is. Maybe the Admin can try to find out.
I wish there were a better solution to protect this community. I love it here!
Admin: If you need any help let me know!