You will have to find this code in register.php.
PHP Code:
// check for errors
if (!empty($userdata->errors))
{
$_REQUEST['do'] = 'register';
$errorlist = '';
foreach ($userdata->errors AS $index => $error)
{
$errorlist .= "<li>$error</li>";
}
$username = htmlspecialchars_uni($vbulletin->GPC['username']);
$email = htmlspecialchars_uni($vbulletin->GPC['email']);
$emailconfirm = htmlspecialchars_uni($vbulletin->GPC['emailconfirm']);
$parentemail = htmlspecialchars_uni($vbulletin->GPC['parentemail']);
$selectdst = array($vbulletin->GPC['dst'] => 'selected="selected"');
$sbselected = array($vbulletin->GPC['showbirthday'] => 'selected="selected"');
$show['errors'] = true;
}
Then you can either use a if statement in the foreach which will take a file edit or you can try to find a hook in the user datamanager that will do what you need.