Routes for DNS forwarding?

At the moment I have following configuration:

interfaces {
......
    ethernet eth2 {
        address 192.168.100.2/24
        description WAN
        duplex full
        firewall {
            in {
                name WAN_IN
            }
            local {
                name WAN_LOCAL
            }
        }
        ip {
            source-validation strict
        }
        speed 100
    }
    loopback lo {
    }
    openvpn vtun0 {
        config-file /config/ovpn_client/config.ovpn
        openvpn-option "--tun-mtu 1450"
    }
}
service {
    dns {
        forwarding {
            cache-size 1000
            listen-on eth0
            listen-on eth1
            name-server 8.8.8.8
            name-server 8.8.4.4
    }
}

 How I can re-route my DNS traffic via "openvpn vtun0" instead of "ethernet eth2"?

Thank you.