If the only device at the remote site the VPN is being used to access is the device providing the VPN (the DiskStation), can’t you access the DiskStation’s other services (SMB, AFP, web for management) through the DiskStation’s “10 space” ip? My guess is 10.20.22.66 is the DiskStation’s ip within the VPN network, what happens if you try to connect to those services at that ip instead of 192.168.178.101?
If the DiskStation currently only listens to the necessary ports on the 192.168.178.101 ip, I would hope it can be configured to listen on all its ips.