That sounds quite odd, especially since it worked earlier. Works still fine here with the latest GeForce driver, so that’s not the cause of the issue.
Please check that ‘3D-Reconstruction’ is not set to ‘Off’ and ‘Eye Separation’ is set to a value higher than 0.0. These are the two settings that influence the stereo effect.
BTW: vorpX uses two very different approaches for creating Stereo 3D. All supported games have Z-Buffer 3D, many of them Geometry 3D. Geometry 3D offers the most lifelike 3D experience, while Z-Buffer 3D is twice as fast (but has ‘less 3D’). Especially in resource hungry games you should consider the Z-Buffer mode. Smooth and silky 60fps are very important for a great Rift experience.