On iOS after I'm leaving the application (pressed the home button) and opening it's once again after short period of time - camera interface will not be shown. If I'm leaving the application and waiting ~5 seconds and then return back - everething works as expected.
I'm using Unity 4.5.5 with Vuforia 3.0.9.
The solution at this link works perfectly as described, I know this as I implemented it last night in my own app and now whenever applicatioDidBecomeActive is called the check is made for camera access and an alertview is displayed if disabled.
https://developer.vuforia.com/forum/ios/ios-8-xcode-6-and-vuforia-30-compatibility&sort=2
You may check this link for a solution
https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/ar-camera-becomes-black-second-run