A problem that sometimes arises is that of users sending nasty-grams via PM to each other. Unlike some forum software, vbulletin does a good job of keeping private messages private.
If you find yourself in a situation where you need to know the contents of a private message the following two SQL queries can be performed in the back-end of the vbulletin admin area.
select * from vb3_pmtext where fromusername='USERNAME'
The above code snippet (change USERNAME to whoever you are searching for) will pull a listing of all PMs sent by a user.
select * from vb3_pmtext where touserarray like '%USERNAME%'
The above code snippet (change USERNAME to whoever you are searching for) will pull a listing of all PM's sent to that user.
Once again, these two commands are very powerful and should only be used when the need is great. Remember, your forum community places a good deal of trust in you, it is up to you to keep that trust by not abusing the privacy of private messages.