Duplicate the 6in4 interface with different tunlink, or customize it dynamically:
mkdir -p /etc/hotplug.d/online
cat << "EOF" > /etc/hotplug.d/online/20-6in4
NET_IF="wan6"
if [ "${INTERFACE}" = "${NET_IF}" ]
then exit 0
fi
uci set network.${NET_IF}.tunlink="${INTERFACE}"
ifup ${NET_IF}
EOF
https://openwrt.org/docs/guide-user/advanced/hotplug_extras
Add a custom rule to lookup the VPN routing table for a specific destination subnet:
https://openwrt.org/docs/guide-user/network/routing/ip_rules