For iptables, you can route clients from eth1 (lan) or wlan0(wifi) to vpn.
For firewall, you may need to create a different zone for lan and wifi. So you have to set up two LAN network in /etc/config/network. Things get a little complicated.
If I use iptables or firewall to setup up two different zones (wifi and lan) what happens to DHCP? Do the two different zones have different subnets? Does that mean I have 2 dhcp instances?