Hi,
As we have done in our DEMO you should use "VM Frontpage" as your joomla homepage.
So you will be able to show the categories in your joomla homepage.
If you want to do this by a simple module, then you should look for a module to achieve this. The official "VM Category" module won't work you, since it just list the categories (it is usefull to list the categories on the left or right hand sides, as sub-menu)
What you need is a module something similar to this:
www.phoca.cz/phoca-virtuemart-category-module
Regards.
Support & Development