Funcionou assim:
iptables --append FORWARD --in-interface eth1 -j ACCEPT
iptables --append FORWARD --in-interface eth0 -j ACCEPT
iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE
iptables --table nat --append POSTROUTING --out-interface tun0 -j MASQUERADE
echo 10 vpn >> /etc/iproute2/rt_tables
ip rule add from 192.168.2.0/24 lookup vpn
ip route add default via 192.168.2.1 table vpn
192.168.2.1 é o gateway local pelo qual desejo rotear, 192.168.2.X minha rede local.