Adding VPSes to a private network

Having the possibility of creating a non-routed private network to a number of VPSes would be lovely.
Allocating an RFC1918 subnet of an arbitrary size and then connecting it to another NIC on select VPSes in a dedicated vxlan would be greatly appreciated. It would save us from having to fight as hard securing our traffic on public interfaces, and could potentially even save us some traffic cost if the traffic is just internal.