Is this setup ok?
 

Say I had a forum eg. forumcategories.com. That forum would have an overview of different categories with eg. gardening, shopping etc.

Then I have say garedeningcategory.com with just the gardening category.

Is this legal or no?

To take the vBforum as an excample:
www.forumcategories.com would display the entire forum http://www.vbulletin.com/forum/forum.php
And www.gardeningcategory.com would display http://www.vbulletin.com/forum/forum...ales-Questions

*Your forum is accessable from multiple domains but each domain shows a different forum style. This is OK and does not require multiple licenses.

*Your forum is accessable from multiple domains and your forums are configured to show/hide certain forums based on the domain. This is not OK and requires an active vBulletin license for each domain. In this situation the forums appear to have separate content thereby making them separate forums in the eyes of the license agreement. Your forum content needs to be the same for each domain in order to qualify as a single forum.