[Dismounted]

You may as well just turn on the No-Cache header option in vBulletin Options. If you do insist on using your method, you need to return the value and not print it.

PHP Code:

function autoVer($url)
{
    $path = pathinfo($url);
    $ver = '.' . filemtime('/home/yadayada/public_html/' . $url) . '.';
    return $path['dirname'] . '/' . str_replace('.', $ver, $path['basename']);
}

$bannercss = autoVer('/css/banner.css');