AWS Device Farm seems only able to access public hosts/ips. I am looking into testing in non-production environment, but I could not find the settings to allow it to access hosts in a particular VPCs.
I work for the Device Farm team.
Currently, there is no out of the box support for VPCs with Device Farm. However, one of the ways to achieve this would be to whitelist the IP range of devices in device farm and have your VPC host accept the connection from this IP range.
Hope that helps.