I'm trying to work through this one myself, or perhaps it's only superficially similar. I *can* run Vuforia within Unity 5.5.0f Windows 64 bit, and Play Mode *works*. However, when I attempt to build for x86 or 64, the resulting application cannot initialize the tracker or load data sets.
"Could not initialize the tracker"
"Dataset XYZ could not be loaded and cannot be activated"
I first attempted to debug the latter, which led me to modify the dataset paths, etc, but without success. I believe the former is the show stopper - for whatever reason, Vuforia in a Win x64 build (outside of Unity) is not working for me.
You can explicitly 'turn on' the webcam (by grabbing WebCamTexture.devices.name, putting it into a WebcamTexture, etc), but that doesn't solve the problem. I've seen this behavior both within our local project and the official vuforia sample set (works in Unity Editor, Android, and OSX Editor, but fails in deployed Win applications).
Another data point - my coworker initialized our project environment, and he programs in OSX. I wondered why my webcam wouldn't work in Play mode in Windows, but after re-importing the Vuforia asset package (6.2.6, seemingly no meaningful changes) Unity Editor Play mode *did* work. Could be unrelated.