I use Joomla 3.4.5 and Virtuemart 3.0.12 and even though many bugs have been fixed in the latest VM edition, I observed a strange behaviour in thumbnails replacement option. I usually use dynamically created thumbs of size 160x120 for some products and I replace other thumbs with ones of size 90x120. The matter is that VM keeps renaming my replaced thumbs and add the suffix _160x120. Even more, every time I change something in a product and then save it, VM 3.0.12 replaces my replaced thumbs with the dynamically created ones. Very weird.
See http://docs.virtuemart.net/tutorials/templating-layouts/222-different-thumbnail-sizes.html
The matter is that I need different thumbs for different product categories and not for different pages.
http://forum.virtuemart.net/index.php?topic=132387.msg457905#msg457905