Category module : How to show only top level until cat. with subcat is chosen

[iamalive]

Hi

I'm using VM 2.0.20b and Joomla 2.5.11

The standard VM Category Modul works as described, showing 2 levels. However, I would like to only show the top level until someone clicks a category which has a sublevel. So the basic setup would be a single category level. How can I change the module to work like this?

You can see the site here: http://umwelttechnologien.ch/

Thanks for any help and ideas.

David