I actually got in to some trouble with this a few minutes ago and Lynne was kind enough (as always!) to point me I'm using process_tempaltes_complete instead of parse_templates.
With that help, now I am using this in my codes:
PHP Code:
if ($vbulletin->options['storecssasfile']) {
$template_hook['custom_css_links'] .= '<link type="text/css" rel="stylesheet" href="' . vB_Template::fetch_css_path() . 'MY_CSS.css' . '?d=' . $style['dateline'] . '" />';
}
else {
$template_hook['custom_css_list'] .= 'MY_CSS.css';
}