Forcing macOS Sierra L2TP client to use specific interface
Having two default gateways in macOS will result in: the gateway of the interface with the higher priority will be the default one and the other one is disregarded.

To get the order enter in Terminal: networksetup -listnetworkserviceorder.

So remove the default gateway of interface en8 and either add a custom route to the VPN server:

sudo route add -host &lt;VPN-server-ip-address&gt; -interface en8


or

sudo route add -host &lt;VPN-server-ip-address&gt; -link &lt;MAC-of-en8&gt;


or to the network (here the example network: 10.0.0.0/16)

sudo route -n add -net 10.0.0.0/16  172.20.10.1




Depending on the network environment of the VPN-server a second inverted route pointing to your local 172.20.10.0/28 network has to be added there.