Edit your config.php file and make sure you have a valid email in this line:
PHP Code:
$config['Database']['technicalemail'] = 'dbmaster@example.com';
Then, go back to your site and produce the database error and let us know what it says in the email you get. We need to know the exact error that is emailed to you in order to help out. (Most likely, on a fresh install, you have some wrong information in the config.php file and vbulletin is unable to connect to your database.)