Did you verify that your backup was complete? (check the last lines of the export file).
The only reliable method of backing up and restoring a database is with shell access via telnet or ssh. This is because backing up with a PHP script like that in the Admin CP or phpMyAdmin can result in PHP timeouts errors and an incomplete backup file. Please see the instructions here:
Backup:
http://www.vbulletin.com/docs/html/m...nce_ssh_backup
Restore:
http://www.vbulletin.com/docs/html/m...ce_ssh_restore