Please help me to separate vpn and wan traffic via two different ethernet ports

Installing and Using OpenWrt Network and Wireless Configuration
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uci set network.route1="route"
uci set network.route1.target="0.0.0.0/0"
uci set network.route1.gateway="10.10.1.1"
uci set network.route1.table="100"
uci set network.rule1="rule"
uci set network.rule1.src="192.168.1.187"
uci set network.rule1.lookup="100"
uci commit network
/etc/init.d/network restart

You can also configure it via web interface.