OOB

I am curious what is the best practice for OOB for a core infrastructure environment. Obviously, there is an OOB kit for customer managed devices via POTS, Ethernet, etc ... And there is OOB for core infrastructure typically a separate basic network that utilizes diverse carrier and diverse path when available. My question is, is it best practice to extend an inband VPN throughout for device management functions as well? And are all management services performed OOB, e.g network management, some monitoring, logging, authentication, flowdata, etc ..... If a management VPN is used is it also extended to managed customer devices? What else is can be done for remote management and troubleshooting capabilities? Mike