Here are the pics of those hungry little creatures and the damage they've done.

You have a nice hoard of Achemon Sphinx caterpillars. At this stage, I'd suggest that some hand removal is called for to eliminate as many of them as possible. During this procedure, you can see if there are eggs and/or smaller caterpillars, too. If so, you may want to apply Bacillus thuringiensis for caterpillars (Bt-K). Bacillus products aren't helpful on mature caterpillars.

These turn into very attractive moths, by the way ....in case you are interested. ;-)

I was worried about pests on my grapes before they gave me my crop, but now that it's mostly jelly on the pantry shelves, a little defoliation wouldn't seem like such a major issue. My worry this year was grape flea beetle. I had given a garden talk last fall, and none of the gardeners in the group had a crop of grapes. They blamed it on a frost, but we didn't have a killing frost, lol. They just never looked for the flea beetles and didn't recognise them as being a problem, since it was a new phenomenon for our area. This summer I nipped them in the bud before damage was done and had a lovely crop. IOW, try to determine the damage potential to the plant. Some things are worth the effort for a full scale eradication and some not.

Hand removal should suffice if you think they need removed. Unless the defoliation is over a large area of vine, it likely won't even hurt them in the long run. I grow aristolochia on trellises just for food for the pipevine swallowtail cats. They have a ball. The vine returns just the same every spring like nothing has ever happened, and I get some lovely butterflies all summer.

Thank you both for your response.
rhizo - After the initial shock of leafless vines wore off I googled 'Achemon Sphinx caterpillars'. I had no idea that these little guys turn into a moth similar to the Hummingbird Hawkmoth that I love to see around.
calliope - Yes, a bit of defoliation at this time of year is not a disaster. If these guys need it they can have it as long as this won't affect the growth next year.

I will let them live...my dogs on the other hand are fascinated with them and are picking them off and playing with them.

Are they poisonous to dogs?

There are some 100 different Sphinx moths in the USA and not all should be called "Hummingbird" moths since only one of the "Clearwings" is the "Hummingbird" moth. I am surprised at the number of University web sites that do combine all Sphinx moths into the "Hummingbird" moth category. The larva of yours will not become "Hummingbird" moths.

All's fair in love, war and common names. If it makes you crazy when someone uses a common name in a way different from the way you use it....well, see ya at the booby hatch. Besides, it was compared specifically to the hummingbird hawkmoth. Macroglossum stellatarum is refered to as a "hummingbird hawkmoth" commonly enough that it's a perfectly grand common name. It ain't taxonomy, just roll with it.

builderbabe, just to clarify...the Achemon moths DO resemble and are often mistaken for hummingbirds. I often call them 'one' of the hummingbird moths. Don't want you to stop using your common name for these great critters.

As far as the defoliation goes, here's my thoughts (for what they're worth, ha). If these caterpillars were grazing on an annual plant at this time of year, I'd say let them be. But, perennials and woody plants are hard at work packing away carbs for winter storage at this time of year. Nearly all of the photosynthates manufactured in the leaves are stored as the energy source to get them through the winter, as well as supplying the only source of fuel for the new growth in the spring!

In my thinking, allowing your grapes to be picked clean now means that they won't be able to face the spring with a full 'bank account' of energy resources. That could mean less productive plants next year. Does that make any sense, lol?

A plant with little in cash reserves has to go into savings account, which always means stress, dieback, disease problems, etc.

So, you might want keep the caterpillar damage limited until later in the fall. Your plants still have lots of growing season left.

Rhizo - Nice analogy.

To all you knowledge filled readers - So what is a fair way to rid my balding vines (they've only got hold of 2 out of 16 so far) of these chubby guys?

I'd be very tempted to try Neem oil on the plants. It acts as an anti-feedant. It probably won't kill a bunch of caterpillars outright, but is likely to prevent them from feeding and moulting. Apply it to the plants you are trying to protect.

I've not used Neem oil for caterpillars myself, but it is labeled for their control. I've had such good luck with it that I can't help but think that it would be a worthy experiment.

If you are still finding caterpillars late in the season, don't worry about 'em. By then, your grape plants will have plenty of 'cash in the bank', lol!