You can set the menu module up to only show sub categories by putting the "Level Start" option at "2" (without quotes) under the Module Parameters. This will then only show the menu at the second level (not main level categories, but 1st set of sub categories).
You will also want to make sure "Always Show Sub Items" is set to "No" under Advanced Parameters.
As I recall set, "Show Root Menu Item" to "No" as well under Advanced Parameters.
From there, the menu will show accordingly, only showing the sub-menu of the active main category selected.
HOWEVER, this does not prevent the Module CONTAINER from loading (the div in which the menu mod would load), as that is Joomla dependent based on pages. I suggest, if you have what I would expect to be a blank module loading on pages where no category is produced on Joomla pages, that you look into metamod. Use metamod to load my menu module only on Virtuemart Category pages with a small php script.
http://www.metamodpro.com/