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.

However...

..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).