I've been trying to follow this guide (using Luci) to use NordVPN on my router, however the guide is for router wide VPN if i understand correctly. I wanted a single device to use the VPN.
I looked around and found PBR (Policy-Based Routing) can make this possible and installed the Luci version, however I can't quite figure out yet how to put two and two together. Has anyone done this or anyone has any idea how to do it? I can't quite find a clear cut guide for Luci that I can follow 
Did you read the PBR guide? Basically, you'll setup the single device to use the tunnel while the others bypass it:
https://openwrt.org/docs/guide-user/network/routing/pbr
However, if you are only using literally a single device over the tunnel, is it an option to put the VPN directly on said device? This should be possible (and easy) for most computers, phones, tablets, etc.. But obviously this would be difficult or impossible for many STBs/game consoles and other devices of that nature.