Before Virtuemart 3.2.6 (http://virtuemart.net/news/482-virtuemart-3-2-6-security-release-and-overhauled-infrastructure) we used the layout order_done. There was the request to be able to create a hidden menu item for the order done page. Adding that we noticed that "order_done" is generically reserved by joomla as sublayout "done" for the layout "order". For that reason, we had to rename the order_done to orderdone. The new name reflects that it is an own view. There is a fallback in place, so you should delete the old order_done.php. Due template overrides, old layouts are not automatically deleted.