Yes Peter,
you are right, even the simple target="_parent" solve the problem, and links will open the parent window instead of in the lightbox.
For the notification box that shows off after the product is added to cart (inside the lightbox) the same target="_parent" must be added to "continue shopping" and "show shopping cart" links. And those links are generated by this PHP file:
/components/com_virtuemart/controllers/cart.php
in 117th, 118th, 123rd and 128th lines. Basically as you mentioned; adding target="_parent" to all of these <a href....> element everything will be good to go.
BUT KEEP THIS IN MIND. When ever you upgrade your Virtuemart, this change will be retrieved, because cart.php will be overwritten, so you should do this change every time you upgrade/update your Virtuemart.
Regards.