Routing takes place on layer 3 (ip) so destinations are ip networks and hosts.

Each packet you create has a destination IP. Your computer looks at your route table to see where it goes by matching the destination ip with each network. It will be sent to the most specific match first and your default gateway last.

If you’re default gateway is you’re vpn server via your vpn interface then you just need to add more specific route for destinations of interest through a different gateway (you’re router) via the physical interface

Raw disk access is a privilege in Linux, usually reserved for root.

You could have root change the permissions on the directory to allow another user or group write access.

Yes that’s called routing.

You don’t bind it to a NIC, you specify the destinations you want forwarded to each interface. Your VPN connection is just another interface.

If you’re looking for good docs, you may want to Google split tunnel vpn, and also bone up on your networking.

A few static routes should get you what you need

The majority of the Internet’s routing and switching architecture is BSD based. Historically it had the most stable and performant network stack of all the OSs.

I used it extensively at one job in a previous life when I was a network appliance developer. It was rock solid and lightning fast. Tried it as a desktop at home and had a terrible experience.

The little differences in the Unix commands used to drive me nuts as well…