>>When I try to purchase the membership product from my sales page, I'm getting this error message:

In order to accept POST requests originating from this domain, the admin must add the domain to the whitelist. <<

This error is being generated by VBulletin.

Pls add this to your forums/includes/config.php file:

define('SKIP_REFERRER_CHECK', true);

Then re-test and see if it still occurs.