Nested VPNs on iPhone iOS 11
I have received a response from Apple Support:


  I talked to VPN Engineering about your goals and, alas, what you’re
  trying to do is just not possible. We specifically prevent traffic
  from a VPN provider from going through some other VPN interface, so
  your per-app VPN provider — be it a packet tunnel provider or an app
  proxy provider — won’t be able to route traffic over the IKEv2 VPN
  interface.


As of iOS11, this is not currently possible.