Here’s a very nice recollection of bare feet on Martha’s Vineyard.
It’s entitled Going Barefoot — An Island Specialty and chronicles how so many went barefoot there during the 1960s, including pretty famous folks like Art Buchwald and Mike Wallace.
It also pretty much reinforces what we’ve already figured out—that NSNSNS signs only really appeared during that time. It took longer on Martha’s Vineyard, but it came nonetheless.
I do believe Island retailers – the majority of them off-the-grid counter culturists themselves; why else would they be here?! – were late to tumble to the what-the-hell? attitude of merchants on the mainland who were busy tacking up NO BARE FEET signs. But eventually they caught on, and gradually hippies grew up and accepted jobs in the banking industries and bought Beemers and put on shoes (not necessarily in that order).
I suspect this is typical of other resort-type areas with water around. They managed to resist the shoes-required wave for a while, but eventually succumbed.
To our, and their (I believe), detriment.
Ahcuah 
Great article, I grew up on L.I. NY. I have for a long time forgotten that. I now remember those days in spring when we would peel our shoes off for the warm months ahead, had to break in our soles for the first week or so to get them toughened up for summer. It would hurt for a short time, but we never ever let anybody know we were in pain. But by the end of October we had the toughest soles and could walk and run on anything. Those were great times growing up in the 60s – 70s.
As a kid and teen, before I emigrated across the oceans to NZ, I spent many summers on Cape Cod and usually they included a trip or two to Martha’s Vineyard. One of the things I most loved about the place was that it was the only place I’d ever been in the US that I saw people walking barefoot in public, in town centres, into shops and restaurants. Now this was the 90s and early 2000s so it wasn’t by any means overwhelming numbers, but it was enough to be noticeable. Even in summery Cape Cod bare feet were for kids only and confined to residential neighbourhoods. On the Vineyard people walked right through Oaks Bluff, into the movie theater or the shops and the Methodist Village, all without shoes. At the rustic country stores in Chilmark it was the same story, bare feet on the wooden floors.
Now it is true there were many NSNSNS signs around the island, but they were generally ignored, unlike back on the Cape. I remember entering many such locations without an issue. It was the only place as a teen that I felt bold enough to go barefoot into a movie theater or shop. The one exception was Edgartown, the wealthiest village on the island. It was much dressier and I never saw people shoeless there.