[glorify]

Nice idea. Didn't look stock so I redid it. Use it if you want, works and looks great:

Add Underneath in the same spot instead:

<tr>
    <td class="thead">Search Tags</td>
       </tr>
       <tr>
            <td class="vbmenu_option" title="nohilite">
<form action="tags.php" method="get">
<if condition="$session['sessionhash']">
       <input type="hidden" name="s" value="$session[sessionhash]" />
</if> 
   
    <div><input type="text" id="tag_input" name="tag" size="25" class="bginput" autocomplete="off" /><input type="submit" class="button" value="$vbphrase[go]" />
</div>
</form>
</td>
</tr>       
<tr>
    <td class="vbmenu_option">
<div><a href="tags.php$session[sessionurl_q]" accesskey="6" rel="nofollow">View Tag Cloud</a></div>
</td>
</tr>

DEMO (I have a custom navbar, but the search drop down looks default when clicked )

Not good at plugins, but it'd be a great one.