Kerry,

Valiant grape normally should be hardy in your zone, but I guess the problem is because it's growing in a pot above ground, the roots might get too cold.

I have a few plants in similar situations and for winter I just dig a hole in the garden or a flower bed, sink the pot in the ground up to the rim (hopefully it's not too heavy to handle) and then fill in around the pot with loose soil, trying to leave no air pockets. If the vine is growing up a trellis you might have to carefully take down first.

It might not hurt to mulch with leaves or straw and then when it snows to mound snow over that as well.

This will be my first winter with my valiant grape. Mine is growing in the ground so I may just remove from the trellis and lay on the ground, so it can benefit from snowcover.

Glen