Hello double check that you really have no mysql table prefix in vbulletin and your cookie prefix is bb as noted in the config.php file of vb
If you have no table prefix then your issue is on of inputting the wrong forum database info on the initial config section of install
Just rerun install.php and check all information your entering. Basically the install can not connect to your vb
and grab the usergroups info to import into our usergroups table.