Have you tried this endpoint :
getOrganizationApplianceUplinkStatuses
/api/v1/organizations/{org_ID}/uplinks/statuses:
[{"networkId":"X","serial":"X","model":"MX450","highAvailability":{"enabled":true,"role":"primary"},"lastReportedAt":"2022-09-20T19:55:47Z","uplinks":[{"interface":"wan1","status":"active","ip":"X","gateway":"X","publicIp":"X","primaryDns":"X","secondaryDns":"X","ipAssignedBy":"static"}]}]
Its a MX450 and not a vMX tho...
My MX450 in concentrator mode DO report their wan interfaces via getNetworkDevices