The reading of PMs is not a function built into vBulletin, and considerable skill and understanding would be required to do so. However, there are hacks that enable specific usergroups to do this - for example:
https://vborg.vbsupport.ru/showthrea...ivate+messages
If you wish to ensure that PMs cannot be read by staff members then simply don't add the hacks that permit it.
Without using these hacks it's virtually impossible.