you need to edit template forumhome_forumbit_level2_post subfourms part and use vbmenu javascript to list them inside a menu .. using vb3 will be something like this:
HTML Code:
<if condition="$show['subforums']">
<a href="#sub">
<img src="$stylevar[imgdir_button]/collapse_thead_collapsed.gif" border="0" alt="subforum"></a>
<script type="text/javascript"> vbmenu_register("sub_$forum[forumid]", true); </script>
</if>