macOS native VPN can't cifs:// into file server on another network
Use smb:// for file shares and vnc:// for screen shares. I’ve not had good success with cifs:// and Finder. 

If smb fails, check for open ports using network utility and look at the console on Mac and error log on the server/Linux/pc side. Network utility will show you if you have an ip / network conflict. The vpn possibly can’t route any traffic correctly if both non-routable networks conflict. Either the local or remote will have to change NAT range or move to IPv6.