[mokujin]

This is my plugin, install and paste your Google search ID in vB Options -> Message Searching Options, there is a setting for Google Custom Search.
Edit navbar template, edit <form> search to this:

PHP Code:

            <form action="misc.php?do=googlecs" id="cse-search-box" class="navbar_search">
                <input type="hidden" name="do" value="googlecs" />
                <span class="textboxcontainer"><span><input type="text" name="q" class="textbox" tabindex="99"/></span></span>
                        <input type="hidden" name="ie" value="UTF-8" />
                <span class="buttoncontainer"><span><input type="image" class="searchbutton" src="{vb:stylevar imgdir_button}/search<vb:if condition="$stylevar['textdirection'] == 'rtl'">_rtl</vb:if>.<vb:if condition="is_browser('ie') AND !is_browser('ie', 7)">gif<vb:else />png</vb:if>" name="submit" onclick="document.getElementById('navbar_search').submit;" tabindex="100"/></span></span>
            </form> 


I've used this for years ago (I closed my forum a year ago)
Good luck