Yes, link is broken in template file. It should be:

<a href="index.php?option=com_vminvoice&controller=invoices&task=pdf&cid=<?php echo $row->virtuemart_order_id ?>" target="_blank">

(we have error in link in documentation on our website).

If you change the link is changed and problem is still there, it can be also in layout override of your template. In that case, check also if there is file templates/TEMPLATE_NAME/com_virtuemart/orders/list.php.

Thanks!
I dont know why, but it works now.