Do you want to limit maximum recipients to send PM at a time. You can set this very easily in vBulletin forum. You can also set different number for different usergroup.

This can be done by editing usergroup permission in admin control panel.

Login in admin control panel >> Usergroups >> Usergroup Manager

Select "Edit Usergroup" and click "Go" button of user group you want to limit number of recipients to send PM at a time.

Scrool down, and set the limit in "Private Message Permissions".

Finally click "Update" button at bottom of the page.