Try
<vb:if condition="'REQUEST_URI'=='index.php'">
Ad code here.
</vb:if>
Where it says index.php you must put the full file path to the root of your forum, etc if my forum was
http://lololol.com/iamaforum/lolol.php
It would be
<vb:if condition="'REQUEST_URI'=='lolol.php'">
Ad code here.
</vb:if>
I'm pretty good with vbulletin, need anymore help goto mortalcoders.com and pm me there.