[calorie]

To disable most logins, you could add a plugin to the login_verify_success hook:

// untested, you need to test it out !!
// 1,2,3 are userids permitted to login

if (!in_array($vbulletin->userinfo['userid'], array(1,2,3)))
{
	$return_value = false;
}

Make sure not to block yourself - use a test account to test it out.