The non meraki vpn is just one configuration. So all devices in the org will try to set up the vpn to that peer.  So if the 3rd party side also configure to connect the z3 it should work.   "Configure which MX or Z1 units in your Organization will connect to this peer. When "All networks" is selected, all MX and Z1 units will connect to the peer. When a specific network tag or set of tags is selected, only networks that have a listed tag will connect to the peer." ... View more