Have not tried it but possibly this will work as the code you need. you may need to edit/tweak it a bit for your site.
Code:
<table cellpadding="4" cellspacing="1" border="0">
<tr>
<td class="thead">$vbphrase[search_forums]</td>
</tr>
<tr>
<td class="vbmenu_option" title="nohilite">
<form action="search.php?do=process" method="post">
<input type="hidden" name="do" value="process" />
<input type="hidden" name="quicksearch" value="1" />
<input type="hidden" name="childforums" value="1" />
<input type="hidden" name="exactname" value="1" />
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
<div><input type="text" class="bginput" name="query" size="25" tabindex="1001" /><input type="submit" class="button" value="$vbphrase[go]" tabindex="1004" /></div>
like I said I did not test it, but its the standard code for the search box so it should pull from all threads/forums/posts etc.