Actually, that not quite true, Marco. We are able to use small snippets of vbulletin code in our hacks. For example, my show spiders on the forumhome page snags code from online.php, moves it to index.php and then refines it. As long as we don't use huge amounts of code, we are allowed to redistrubute it.
I know cause I asked before I released mine
The one thing that throws a monkey wrench in the works is the selling the hack. Because it wouldn't be sold here where all users are guaranteed licensed, I am not sure how Jelsoft would feel.