SoftEther VPN User Forum

Did you try that already? I think it should just work out of the box.

Assume you are connected via L2TP. Basically what happens is that the PI takes every packet intended for X/1000/udp and repacks it into a new packet called Y/1701/udp. When the packet arrive to Alice, the reverse of this operation will be done.

But to be really sure, I'd make the VPN server a standalone with no other services running on it, not even X

Please try to install SoftEther VPN client on Alice and connect to local VPN server.
After that, please assign an IP address to the virtual NIC manually.