Hi, the reason for the breadcrubs problems can be that you have not yet set a default "master" link to Virtuemart. You should have done this when you set up vm in the beginning. If you have not, the system might have done this for you, and thus make everything else in relation to this appear in the breadcrumbs when they shouldnt.. I have had this problem before as well,, its VERY annoying:)
If this is the problem, read the first symptom in this forum post. It should fix things for your:
http://forum.virtuemart.net/index.php?topic=69544.0
If this is not your problem, I would recommend that you check if the joomla menus items are all linked to your vm categories, and if yes, is this really necessary? VM works better without any interference from the joomla. On my page i have set the main manu, the bar on the top, to link to articles where i have a introduction to each product category(lets say an image and a headline), and then i have added the VM categories to these pages, woking 100% alone, not with the joomla menu items. If you want to do this i recommend to use meta module (great extention) to be more flexible with vm in the joomla enviroment without linking with the menu items.
Last, if you do choose to link joomla menu items with vm categories, make sure that the names are the same in the two systems. If not, joomla breadcrumbs and vm breadcrumbs will be confused an there is going to be random probems in breadcrumbs, and in the url..
Hope this has been a bit helpfull.. I have had this problem myself...