This is not a tested solution (use at your own risk), as I do not have threads and sub-forums under the same forum.
In your FORUMHOME template:
Find and cut:
<if condition="$show['forumslist']">
<!-- sub-forum list -->
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" style="border-bottom-width:0px">
<tr>
<td class="tcat" width="100%">$vbphrase[subforums]<span class="normal"> : $foruminfo[title]</span></td>
<if condition="$show['forumsearch']">
<td class="vbmenu_control" id="forumsearch.subforums" nowrap="nowrap"><a href="search.php?$session[sessionurl]f=$foruminfo[forumid]" rel="nofollow">$vbphrase[search_this_forum]</a> <script type="text/javascript"> vbmenu_register("forumsearch.subforums"); </script></td>
</if>
</tr>
</table>
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<thead>
</thead>
$forumbits
</table>
<br />
<!-- / sub-forum list -->
</if>
Still in the FORUMHOME template:
Find
<!-- controls below thread list -->
Above it paste:
<if condition="$show['forumslist']">
<!-- sub-forum list -->
<br />
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" style="border-bottom-width:0px">
<tr>
<td class="tcat" width="100%">$vbphrase[subforums]<span class="normal"> : $foruminfo[title]</span></td>
<if condition="$show['forumsearch']">
<td class="vbmenu_control" id="forumsearch.subforums" nowrap="nowrap"><a href="search.php?$session[sessionurl]f=$foruminfo[forumid]" rel="nofollow">$vbphrase[search_this_forum]</a> <script type="text/javascript"> vbmenu_register("forumsearch.subforums"); </script></td>
</if>
</tr>
</table>
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<thead>
</thead>
$forumbits
</table>
<!-- / sub-forum list -->
</if>
That should work for you.