[Scanu]

So you're trying to replace this

<!-- message -->
        <div id="post_message_$post[postid]">
            $ad_location[ad_showthread_firstpost_start]
            $post[message]
        </div>
        <!-- / message -->

With this?

<div class="red">
<!-- message -->
        <div id="post_message_$post[postid]">
            $ad_location[ad_showthread_firstpost_start]
            $post[message]
        </div>
        <!-- / message -->
 </div>

I don't know how should be on vb3 but try this

PHP Code:

$replace = '<!-- message -->';
        $vbulletin->templatecache['postbit'] = str_replace($replace,'<div class="red">'.$replace,$vbulletin->templatecache['postbit']);
        $vbulletin->templatecache['postbit_legacy'] = str_replace($replace,'<div class="red">'.$replace,$vbulletin->templatecache['postbit_legacy']);
$replace = '<!-- / message -->';
        $vbulletin->templatecache['postbit'] = str_replace($replace,$replace.'</div>',$vbulletin->templatecache['postbit']);
        $vbulletin->templatecache['postbit_legacy'] = str_replace($replace,$replace.'</div>',$vbulletin->templatecache['postbit_legacy']);