To find the template to modify, do this - vboptions > General Settings > Add Template Name in HTML Comments > set to Yes . Then go back to your page and view the source code and you will see the name of the template called around your part of the code. (Sometimes the template is the one mentioned at the very top of the page source.)
I'm guessing you are talking about the FORUMDISPLAY template.
I'd guess you'd want something like (in a couple places):
HTML Code:
<vb:if condition="$foruminfo['forumid'] == x">
your link
<vb:else />
<a href="newthread.php?{vb:raw session.sessionurl}do=newthread&f={vb:raw foruminfo.forumid}" rel="nofollow" class="newcontent_textcontrol" id="newthreadlink_top"><span>+</span> {vb:rawphrase post_new_thread}</a>
</vb:if>