[MarkFL]

Try this...

Create a plugin hooked at "notices_noticebit" with the Plugin PHP Code :

PHP Code:

if (strpos($notice_html, 'Birthday') !== false)
{
    $notice_html = str_replace($vbulletin->userinfo['username'], $vbulletin->userinfo['field5'], $notice_html);
} 


Save it with "Plugin is Active" set to "Yes"...this should do the trick.