How the FOV can be adjusted varies from game to game. A neat trick that both the Vive and the Oculus CV1 use to gain a few extra pixels where it counts most. The bottom bar is larger than the top bar because the Vive uses a slightly asymetric view to account for the human eye being better at looking downwards than upwards.
The image is perfectly centered vertically, i.e. That also requires a high enough field of view (120° horizontal for Vive), otherwise the image will look distorted. If you see bars at the top and/or bottom, you need to set the Image Zoom value to 1.0 and the aspect ratio mode to “Pixel 1:1” on the image page of the ingame menu.
