VPN over ethernet, Internet over wifi I use a VPN to connect to my work network. It works fine for accessing internal resources, but getting out to the internet over the VPN can be spotty. (Not because sites are blocked. It's just slow.)

Is there a way to segment traffic so that VPN works over the ethernet connection but access to the internet goes over wifi?

I don't have a lot of experience with networking but I'm comfortable with the Terminal and shell scripting.
Depending on the VPN client being used, this may or may not be possible. Certain VPN clients (like Junos Pulse, for example) can be set by your network administrator at work to enforce certain routing policies when connected via your VPN client. If your sysadmin at work set your VPN client to force all outgoing traffic to go through your VPN then you're most likely out of luck (I know that Junos Pulse can be set to automatically disconnect if the routing table is touched once connected). As your sysadmin at work how it's configured.