localhost uses the reserved loopback IP address 127.0.0.1.
In general a MySQL server runs on the same server as your web server. You can enter in the vBulletin includes/config.php file as host address for the MySQL server localhost. (See example below)
// ****** MASTER DATABASE SERVER NAME ******
// This is the hostname or IP address of the database server.
// It is in the format HOST:PORT. If no PORT is specified, 3306 is used.
// If you are unsure of what to put here, leave it at the default value.
$config['MasterServer']['servername'] = 'localhost';