Check your file permissions as well. It is either that or a missing file. It could be a mod that is missing a file if not the main Vbulll install.
Try disabling your plug ins and see if that works.
Add this to your config.php to the very top - when done just comment out the one for plug ins because this is often used when troubleshooting plug ins so it is good to keep it in there -The top one will help with depreciation warnings. The bottom one will shut your plug ins to allow you to troubleshoot.
PHP Code:
error_reporting(E_ALL ^ (E_NOTICE | E_WARNING | E_DEPRECATED));
define('DISABLE_HOOKS', true);