Proper steps for upgrading
1. Backup all files and database
2. Upload all vBulletin latest version files (3.7.3PL1)
3. Run the upgrade script located at
http://foruminx.com/Forum/install/upgrade.php
4. Remove install.php from the /Forum/install folder
5. If you receive errors, disable plugins via the config.php file edit and use a standard vBulletin skin (the default one with no modified templates)
If you still have problems, then I'd submit a support ticket at vBulletin.com. If you do not have problems, its caused by a faulty modification in which case you'll have to reenable the plugins, and disable/remove them one by one until you find out which one is causing it.