So steam deck is arch based. I just installed arch on my desktop with gnome. I couldn’t get proton vpn to work at all until I installed network-manager-applet. Then it worked fine. (It wouldn’t connect). I had to do a ton of googling before I found that fix, I think a lot of people give up on proton vpn before they find that solution/dependency. It sounds like other packages can provide what proton-vpn is looking for, but I think network-manager-applet gets installed often enough as a dependency where it just works for some people, while for others getting proton to work is a chore.

Now, I don’t know the full dangers of installing that package on the steam deck, maybe someone smarter can comment on that. But I’m thinking installing that package alongside proton vpn is what you need. It should be noted that I installed network manager from Pac-Man and proton vpn from the AUR. I don’t know how the flatpak would behave - but it didn’t work either without network manager installed.

Good luck!