Network namespaces and policy based routing are black magic, IMO.
I’ve got a VPN set up on my router and separate VLANs set up for ordinary traffic and VPN traffic. A device doesn’t need to support VPNs at all, I just connect it to the VPN VLAN and all its traffic goes over the VPN whether it likes it or not. I’ve got separate wifi SSIDs for each VLAN.
My desktop is connected to both VLANs with a network namespace set up for the VPN VLAN, so sudo vpn rtorrent
runs rtorrent in the namespace that’s connected to the VPN VLAN.
My setup is nice, but I wouldn’t recommend it to anyone who doesn’t want to learn quite a bit about networking.
I mean, it’s bits of configuration all over the place that I’ve built up over time. It isn’t a single script on one machine, and you’d need to change a lot of things if you weren’t running Slackware. I can’t really copy and paste it all.