You could do it via conditionals too.
Code:
<if condition="$forumid == X">
header code
</if>
I think that should do it. You could use a array if you want to use a header for multiple forums, or a else considtion to show certain headers in a few forums, and the same header in others.
Link to more conditional statements:
http://www.vbulletin.com/forum/showthread.php?t=200894