The vps has a static ip and a reverse ptr which is why I want to relay outbound mail through it. Postfix and opendkim can manage that but I'm not sure how to configure pmg to relay all outgoing mail via the vps.
I also discovered that some mta won't deliver mail to a host mx not on a static ip with a proper reverse ptr and I wouldn't know about it.
Your setup sounds fine, but I was hoping to spare the expense of another vps. I've been trying to get support for a reverse ptr and static ip with my isp, but they seem incapable of creating the ptr record for me. I even wrote to their netadmin@ and apnic.manager@ addresses but received only stony silence.