Today I worked on implementing VR on a game I already have, Target Practice. It was fairly easy, I just downloaded the Cardboard SDK for Unity from here. Imported to the project, put the main prefab in the scene, configured it and it worked fine.
It fit perfectly with the turret Gameplay, so I put the turret as child of the camera and all worked out by itself. I was able to make the turret aim to the direction of the VR camera.
Unfortunately, there is a bug with the Unity+cardboard SDK which makes the game crash on Android, when a scene is loaded, after using the cardboard magnet trigger. So I had to make a standalone apk just for this game, in order to avoid getting crashes on the standard version of Target Practice.
You can download and play the VR version of Target Practice here: https://play.google.com/store/apps/details?id=com.kamicazer.TargetPracticeVR