I used the following tutorial to successfully integrate Unity into an existing iOS native app: http://www.the-nerd.be/2015/11/13/integrate-unity-5-in-a-native-ios-app-with-xcode-7/. It is working fine for non-AR, but the camera functionality in my Vuforia AR scene is not working (loaded by pressing a button in the native app to "show unity" after the app is up and running). The phone askes me for permission to access the camera the first time - and doesn't ask again. However, I only see the background color of my scene - not the live camera feed image. The Unity project works fine in the editor with the webcam, and the example Vuforia projects work fine on the iOS device I'm using (iPhone 6s).
I've searched quite a while in the forums and online and there seem to be many complaints about similar issues - but I haven't seen specifics that are close enough for me to resolve this from the ideas provided.