You can test your light sensor using an app. I know that Elixir works for this. Run Elixir and then tap the sensors tab. You can then test the various sensors on your device.
When using your VTAB make sure that you do not cover the sensor. It is just to the right and slightly below the word Vizio that is at the top of the tablet in portrait mode, near the edge of the device. If your screen protector is covering it you may want to notch it there.