I basically agree with the response Simone got from Nikon. Although acceptable (slightly blurry, with sharpness varying from shot to shot, most shots "usable") results can be obtained at slower speeds, sharpness in my 300 PF shots increases towards faster shutter speeds and 1/400s sounds like a reasonable guideline for consistently high level of sharpness. I prefer a bit faster speeds myself, from 1/500s to 1/800s. At these speeds, VR stabilizes the viewfinder image, resulting in more precisely controlled compositions and better autofocusing and probably also reduces the effects of hand shake on the images. With the 300/4D AF-S, I needed to go to 1/800s or faster for acceptable hand held sharpness even with 12 MP FX (1/500s didn't do it) so there is some advantage to having VR in the lens, in the form of improved sharpness at borderline shutter speeds. To get the advertised number of stops you have to switch to a lower resolution standard i.e. something that you might see in small print at some distance away, or on a typical laptop display without cropping, or on a mobile phone screen.
I have the same experience basically with other lenses, e.g. I don't normally let the shutter speed below 1/200s with the VR 70-200/2.8II since if I do, sharpness invariably starts to fall. However, at 1/200s VR does help, at 200mm focal length; it helps with composition, autofocusing and reduces the effect of camera shake. I'm perfectly happy having this advantage in my VR lenses and think expecting perfect pixel-level sharpness at slow speeds is just unrealistic and a tripod should be used instead. However, if the usage of the image is a relatively low resolution presentation, then shots taken at very slow speeds can sometimes be considered acceptable.