http://isdb4l77sjqoy2qq7ipum6x3at6hyn3jmxfx4zdhc72ufbmuq4ilwkqd.onion/docs/guide/app-settings.html
If you use this option, you can enable "Disable Orbot for non-onion traffic" option in Orbot settings making the rest of your device traffic to not use Tor. Please note VPN on iOS can send some traffic to the usual network, if, for example, VPN app crashes.