I would have to specifically see how you have things setup because from what I am seeing you are allowing guests to view that category and ads.
Your integrating with vbulletin and the guests group would be the very first group in the usergroups list. If that one is unchecked from viewing permissions under edit categories for the category your talking about then that category would not be shown
You can send us an admin login to look at your setup to see you have it set correctly.
The only way to get the result you have is to not be setting permissions correctly.
This comes right from the showcat.php file
Content visible to verified customers only.
What that code tells me is you have set permissions so that guests usergroup 1 can view that category.