As long as AWCOUPON can be used with VirtueMart, may be you will allow something like:
/index.php?option=com_virtuemart&page=shop.cart&add_id[]=270&qadd_270=10&cid=1
where cid=1 is the awocoupon id (or code as you like)
function onAfterRoute() {
....
// you can filter it to VM pages only:
if (JRequest::getCmd('option') == 'com_virtuemart')
{
// will get autocoupon=1 from the url
$x = JRequest::getVar('autocoupon', 0);
if ($x)
{
// if you want a particular coupon, you'd have to query it by
// $dbj=JFactory::getDBO(); $q = 'select coupon_code from #__something where id = '.(int)$someid..... $db->setQuery($q); $cc = $db->loadResult();
if (empty($_SESSION['coupon_code']))
$_SESSION['coupon_code'] = 'your awo coupon code';
}
}
// thus it will automatically be used in the cart
}