another example:
--------SUPPORT (Category)-----

--
----forum1
----forum2
----forum3
--------TALK (Category)-----

--
----forum1
----forum2
----forum3
----forum4
the category rss

=so for the forum 1-4 is an link to the feed like:
HTML Code:
http://www.vbulletin-germany.com/forum/external.php?forumids=119,120,121,117
(you have to input your forum IDs in the link ,to get category rss link)