This is the problem of Joomla+Virtuemart, we have used "official Joomla breadcrumb module" in StyleMart quickStart package.
Generally in Joomla; breadcrumb hierarchy is built by looking at the menu-items tree. But Virtuemart is giving signal to breadcrumb module to build based on category. (because not everybody is creating menu=items for each category). The result: conflict and confusion!
I would recommend you to use "External URL" for creating menu items for VM related pages, so Breadcrumb won't be generated based on joomla menu-tree (only Virtuemart will be responsible to create the hierarchy based on VM category tree)
Another solution is to use 3rd party "Virtuemart Breadcrumb module" that were developed specifically for Virtuemart (use google.com to find them), they are much more accurate.