Hello,
i am intéresse too to this answer...
When you make an order, the last page after order is the "orderdone.php"
You can find it in : /YOUR_ROOT/components/com_virtuemart/views/cart/tmpl
I see we have an order_done.php, but when i try to put an order, with "pay later", the page is orderdone.php. I have not tested with PayPal paiement, but i think this is the same page.
For me, i have make html for overwriting, so it is : templates/best_theme/html/com_virtuemart/cart
I think it is in this page we have to put the code, but i dont know how, because in the Adwords it say to copy in the Head tag, but in this file i dont see head tag...