Prior to 4.2.3 you cannot do it, SES requires you to use the 'tls://' stream rather than 'tcp://' but this isnt available in older vbulletin versions.
In 4.2.3 a hook (mail_smtp_send_before) was added - which you can set [& use] to update the value
PHP Code:
$stream = ($this->secure == 'tls' ? 'tls://' : $stream);
In 4.2.4 the code has been updated slightly again to provide a new 'TLS Native' option that will use the tls stream value.
4.2.4 (Beta 1) is due out before the end of the year.