The VR effect reduces vibration, it does not affect the amount of light entering the lens. The exposure modes base their choice of settings only on the light entering through the lens So F5.6 is still f5.6 and 1/125 sec is still 1/125. it is just the vibration that is reduced to a level equivalent to using much faster shutter speed.
This means you can take chances in low light and shoot handheld without getting blurred images at much slower shutter speeds than with a nonVR lens - but it is not a magic bullet. it will not compensate for subject movement, only camera shake and the claimed "up to 4 stops" is actually more like 1 or 2 stops in practical use.