When you set up Virtuemart as a component on a menu, the "internal" URL of the page looks something like:

http: //example.com/index.php?option=com_virtuemart&Itemid=83

Even if it's the front page and the URL that the public sees is "http://example.com/", the internal URL contains the option and Itemid information.

When you click on any other Virtuemart link, you get a URL like:

http: //example.com/index.php?option=com_virtuemart&page=shop.product_details&flypage=flypage.tpl&product_id=1&Itemid=74

Therefore, I'd write a rule that checks to see whether $option is "com_virtuemart", and no other categories or pages have been requested.

e.g.

if ($option == "com_virtuemart" && JRequest::getVar("page") == null && JRequest::getVar("category_id") == null) return 101;

(replace 101 with the module id of the module you want to display)

Hope that helps,
Stephen