In reported posts the prefix was not displayed, although I've modified class_dm_threadpost.php.
For those of you who have this problem too, here is a the plugin code that fixes this problem.
prodcut: Thread Prefixes
hook: report_send_process
title: Prefix: Display Prefix in Front of Title (reports)
PHP Code:
if (!empty($threadinfo['threadprefix']) AND $foruminfo['allowprefix'])
{
$threadinfo['title'] = unhtmlspecialchars($vbulletin->options['prefixmarkup_start'] . $threadinfo['threadprefix']) . $vbulletin->options['prefixmarkup_end'] . $threadinfo['title'];
}