If both computers are not connected to the VPN, each can access the other via Remote Desktop over the LAN.
If just one is connected to the VPN, they can’t connect — but I expect that.
If BOTH are connected to the same VPN, however, I’d expect RDP to work over the VPN between the two systems, yet it doesn’t.
One possible difference is that one computer is my work laptop, joined to the company domain, and the other is my personal system, which is not on that domain. Could that be important?
These computers can’t ping each other when connected to the VPN either, nor does file sharing function between the two. It’s almost as if the VPN functions as a one-way valve, allowing each system to see other resources on the VPN, but without themselves being exposed as available on the VPN.