The biggest danger from these sub zero overnight temperatures is not damage to the fruit (the sugars in the berries lowers their freezing point far enough below zero to not be a worry) but the vine going "night-night". Two things can happen, the first catastrophic, the second kind of forcing your hand.
The catastrophe is frost - if the water inside the cells of the leaves freeze, the leaves will immediately die. Remember that water when frozen expands; the pressure from the expanding ice bursts the cell walls open and thus instantly kills them. Without leaves the vine can not photosynthesize any more sugars and flavor compounds and the grapes will not mature further. A real bummer if you still have "green" fruit.
The second scenario is that the vine believes it is now winter and starts going into dormancy. This process is less catastrophic since it is slower - leaves will gradually turn yellow, red and then fall off. A vine can still ripen fruit (although at a much slower rate) after 50% of its leaves have senesced and it may take a week or more to get to this point. Enough cool weather and this scenario is inevitable.
So what can you do? In New Zealand, some grape growers may water their vines when there is a frost warning - this is a double-edged sword since the water may prevent the frost from setting but the vine will then take up that water and dilute the flavors that it has spent all summer developing. Not many growers of grapes destined for premium wines would employ this tactic (one would hope).
The much more expensive option is to create wind. In Gibbston one often sees what look like windmills - but instead of harnessing wind to generate energy, these bad boys generate wind from energy. Usually hooked up to temperature sensors that will turn them on anytime that the temperature approaches zero, the wind they create moves enough air to prevent frost from setting in. At Peregrine we don't have wind machines so we need temporary ones - otherwise known as helicopters! Every morning when there is a frost warning, two or three helicopters show up at 4am to fly over the vineyard until 8am or so, again generating enough air flow to stop frost damage. Gnarly stuff!
