Hi,
I'm user of Google ADWORDS and I confirm that the code should be implement in order_done.php (templates/YOUR_TEMPLATE/html/com_virtuemart/cart/order_done.php). IN my case, it's works perfectly even with 4 kinds of payments-> Paypal, Credit Card (ATOS), Check and Bank Transfer.
My Question here is how to improve the google code.
This line
var google_conversion_value = 0;
is to catch the order total amount of the transaction for ADWORDS analytics (and of course, improvement of your ROI....

). Instead of "0" you'll have to put the value of the order.But I don't know how I could find the value (order total).Something like $this->order-total....

Thank's for your help....