It's not even possible. Ethernet interfaces have a link layer (MAC address),
tun interfaces don't. You can bridge Ethernet to
Apart from that: What is the purpose of a VPN? Usually it is to separate legitimate communication partners from the rest. What is the point if someone just needs to connect to the LAN in order to get privileged access?
Under normal circumstances the only reasonable way is to configure the system as a router. You should block everything from Ethernet to the VPN first and then whitelist accesses you want to allow.