localhost is the default host address of a computer on which a service is running.

localhost uses the reserved loopback IP address

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)
//    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'
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