Redirect guest on payments.php to landing page

[vb_mp]

I am wondering if anyone knows how I can redirect guests who hit the payments.php elsewhere? (I want to send them to a landing page that explains what the payments are about - and prompt them to register first).

I've got the redirect code (from a redirect plugin here):

if ($show['guest']){
$vbulletin->url = '' . $vbulletin->options['bburl'] . '/misc.php?do=page&template=mylandingpage';
standard_redirect();
}

Put I can't find a hook for the payments.php start.

Any help please?