VirtueMart creates a thumbnail once and keeps using it – the new size only applies to thumbnails it creates from now on. Delete the old ones so they are rebuilt:

images/virtuemart/product/resized/

Empty that folder (keep the folder itself). The next visit to each category page creates the thumbnails again in the new size. On a big shop, open the category pages once yourself after deleting, so the first visitors don't wait.