Ok, well, the code you need to edit is this:
<!-- header quick search form -->
<div class="vbmenu_popup" id="navbar_search_menu" style="display:none;margin-top:3px" align="$stylevar[left]">
<table cellpadding="4" cellspacing="1" border="0">
<tr>
<td class="thead">$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="$vbphrase[go]" tabindex="1004" /></div>
<div style="margin-top:$stylevar[cellpadding]px">
<label for="rb_nb_sp0"><input type="radio" name="showposts" value="0" id="rb_nb_sp0" tabindex="1002" checked="checked" />$vbphrase[show_threads]</label>
<label for="rb_nb_sp1"><input type="radio" name="showposts" value="1" id="rb_nb_sp1" tabindex="1003" />$vbphrase[show_posts]</label>
</div>
</form>
</td>
</tr>
<if condition="$vboptions[threadtagging]">
<tr>
<td class="vbmenu_option"><a href="tags.php$session[sessionurl_q]" rel="nofollow">$vbphrase[tag_search]</a></td>
</tr>
</if>
<tr>
<td class="vbmenu_option"><a href="search.php$session[sessionurl_q]" accesskey="4" rel="nofollow">$vbphrase[advanced_search]</a></td>
</tr>
$template_hook[navbar_search_menu]
</table>
</div>
<!-- / header quick search form -->