Grapevines are hardy in the ground, but in pots, they could be vulnerable to freezing, so if the temperatures drop low enough for long enough (-5degC day and night for a week or longer) the compost in the pots might freeze solid, and that will damage the roots and likely kill the plants. You can try insulating the pots by maybe constructing some kind of box round the pots and surrounding them, inside the structure, with something like hay or straw, or you can just wait and see what kind of winter we have and just move them somewhere warmer if it turns out to be a humdinger like 2010, to prevent that kind of damage.