I believe most grapes are pruned in late Fall after the fruits are done ripening, but if you're not trying to get fruit this year you can prune any time you want. Perhaps you should consider binding the vines to the wire so they won't fall off. Just be sure to use something that will not cut into the vines as they get bigger.

Also, you may want to post this in the edible landscaping or orchard forums. Grapes are one of the few edibles i don't grow.