Reporting user email is not used by vbulletin because according to your settings, reporting user might not have an email. (eg. when you enable reporting for guests).
But if you are determined to do that anyway, this should do the trick:
Edit includes/class_reportitem.php, find line:
Code:
vbmail($moderator['email'], $subject, $message, true);
replace it as
Code:
vbmail($moderator['email'], $subject, $message, true, $this->registry->userinfo['email']);