[Andreas]

From paypal.php

$sub = $DB_site->query_first("SELECT * FROM " . TABLE_PREFIX . "subscription WHERE subscriptionid = $subscriptionid"); 
$cost = unserialize($sub['cost']);
if ($_POST['tax'] > 0)
{
	$_POST['mc_gross'] += $_POST['tax'];
}

// Check if its a payment or if its a reversal
if ($_POST['txn_type'] == 'web_accept' AND $_POST['payment_status'] == 'Completed')
{
	if ($_POST['mc_gross'] == $cost[strtolower($_POST['mc_currency'])])
	{
		build_user_subscription($subscriptionid, $userid['userid']);

Take a look at the highlighted part. To me this seems like vBulletin does check the amount.