Never tried it, so not really sure. VM is a component, which is linked/accessible via VM's Joomla modules and Joomla menu links. You can make the modules only show via registered, as you stated, and likewise, don;t show your shop menu links (Joomla menu links) but to only registered users.
If someone already has your store URL and manually enter this in though, they would still be able to access, but I would think that is rare.
-F