I have a Nikkor 18-105mm f3.5-5.6G ED VR lens, and a Nikkor 50mm f/1.8 D. The latter is obviously a faster lens and as such better in low light, but (assuming the zoom lens is set to 50mm), which one will actually give me the most usable pictures under practical hand held conditions, considering that the zoom lens has VR?
The zoom will open up to a maximum of f/5.6 at 50mm; the prime is f/1.8. That's a little more than three stops difference between them (f/1.8->2 is about 1/3 of a stop, then there's 2.8 and 4 before getting to 5.6). That assumes you're shooting both wide open, but the difference will be the same if you're stopping down the same amount from wide-open on both lenses for sharpness.
The original 18-55VR has been shown in reviews to give a real-world two-stop margin for hand-held pictures; the VRII seems to manage a real-world three stops or slightly better. So the prime would give you about one stop more useful range than your older-model lens, but it would be a coin toss between it and the newer model if both are shot wide open.
..at the same aperture on both lenses (which you would need to get the same depth of field), the prime lens has no advantage at all. If you're shooting at f/8, then you have one 50mm f/8 lens with vibration reduction and one without. The zoom with VR will allow you to hand-hold at slower shutter speeds than you can with the prime if the aperture setting is the same. And while the "nifty fifty" is a really nice, sharp lens, it happens that the 18-55mm zoom is no slouch either.
The difference, then, has more to do with your depth-of-field requirements than the available light level. If you need a shallow depth of field, you can't quite get there with an f/5.6 lens in any case, VR or not. If you are shooting to get lots in focus, then having an f/1.8 maximum aperture is no advantage (well, except that the viewfinder image will be ten times as bright -- and that can be enough sometimes).
I would take the fast lens over the VR if I was shooting around 50mm. In bright light, the zoom has the convenience of a range of focal lengths. But in low light the autofocus will start to hunt, and VR will be less useful if subjects are moving.
Static situations - they are roughly equivalent. VR would have a slight advantage if (as Stan suggested) both were stopped down to f/8.
Moving subjects - VR is not very useful, f/1.8 lens can take advantage of faster shutter
Autofocus - f/1.8 will be much faster and more accurate in low light. VR does you no good if your lens is constantly hunting to find focus and missing - you'll end up with lens blur instead of motion blur!
I would tend to go with the fast non-VR lens over the slower VR one. As the above posters say, it's only when stopped down to smaller apertures that you get the advantage of the VR.
I shot a pub gig last friday, and god was I glad to have my 24 and 50mm f/1.4 lenses!! Shot mostly around f/1.8-f/2.0 and it allowed me to keep my ISO down to about 800 (and thus control noise) even in the dimly lit room and get shutter speeds fast enough to freeze the drummer and fingers strumming the guitar strings!