It is not feasible to support 3rd party plugins that plug into Virtuemart. If it works without opc then you would need to get a solution from the 3rd party you are using. With that being said, if you are having the exact same problem as the post you are referring to:
Then the solution below should work for you. The developer of rupostel opc emailed it to me
coupon works only at the cart page, but does not work when the order
function processCoupon(&$d, $after_login=1)
if (method_exists($ps_coupon_process, 'init_coupon_session'))
$rate = $_REQUEST['shipping_rate_id'];
$d['coupon_code'] = $saved_coupon;
/// FIX was done in the following line, original:
$_REQUEST['coupon_code'] = $saved_coupon;
$_REQUEST['coupon_code'] = $_POST['coupon_code'] = $saved_coupon;
the reason for this error was that this line returned no coupon codes
even when $d['coupon_code'] and $_REQUEST['coupon_code'] was set in
$multiple_coupons = explode(';',JRequest::getVar( 'coupon_code', '', 'post' ));
I will implement the fix in OPC1 for VM1 in the next release.
As for downgrading, it depends what you were using before, but more than likely you would have to uninstall your current version to install a downgraded one.