In FullVR mode the game‘s camera field of view and the headset FOV have to match, otherwise the image will look distorted. For many supported games vorpX can handle that automatically, but for others you have to do it yourself.
The help that comes with vorpX contains two guides that explain how to do that. the first one is the ‚Essential Hints‘ guide, the second one the ‚1-2-3 Game Setup‘ guide. Read both, the first one is a bit more theoretical, the second is a practical step-by-step guide.