I love my NixOS thing, because I am part of the cult of NixOS, but it's probably something I wouldn't recommend for most people because it was kind of a pain in the ass to get working. The reason I do it now is because it lives on the same box that is my server.
I've looked into Vyos, it sort of reminds me of the Cisco stuff and it looks interesting, but it never seemed sufficiently better than my NixOS thing.