Hello. Point you are probably looking for it administrator/components/com_vminvoice/helpers/invoucehelper.php somewhere before end of sendMail() function.
You can add there something like:
if ($res===true){
file_get_contents('
www.reviewsite.com/set.php?user={user}&connector={connector}&action={action}&targetMail='.$mailTo.'&delay={delay}');
}
Of course on dependance of your server, if file_get_contents will not work, you will need to use cURL library, etc.
Beware, if this file updates during component update, your changes are discarted and you will have to make modification again.
We can make this as paid customization for you. 1 Hour will be totally sufficient, also changes stays after upgrade.