The template should be FORUMDISPLAY.
I believe the id of the new thread button is called newthreadlink_top and newthreadlink_bottom. Search for that in the FORUMDISPLAY template to locate the new thread button.
You can use an if-else statement as follows:
HTML Code:
<vb:if condition="in_array($foruminfo['forumid'], array(50,51,52,53,54))">
show special buttons
<vb:else />
show normal button
</vb:if>