Thanks for the info on the Vivos Mark. I've been curious for a long time – now I think I'll skip them.
I've tried Nike Free and was pretty disapointed. The basic idea is good – a grooved midsole/outsole that allows the flexibility of a shoe with a much thinner sole, while preserving the cushioning. I expect that Nat doesn't need that cushioning, having become accustomed to the low-impact footfall of unshod or barely shod walking. My problem with the shoes was the poor implementation – they build up certain areas of the midsole (arch, lateral forefoot) while scraping out others (heel) in a fashion that more or less destroyed the elegance of the concept (and patent) they're working with.
"Barefoot shoes", aside from being an oxymoron, are pretty hard to find. There are a couple concept designs floating around, but none have reached the production phase that I'm aware of.
A do-it-yourself idea would be to take a pair of quick-drying socks and 'paint' the soles either with liquid latex (which some modern mocassin makers use – durability will be an issue if used offtrail or on a thruhike) or with a mixture of rubber cement and rubber flakes (also used by some mocassin makers and sold in tins by one of them – I'll try to find it and post). These may be wacky options, but I can't think of any manafactured products to recommend, and I've looked.
Regarding cushioning, the Journal of Rheumatology published a study last year looking at the difference in joint loads experienced by people wearing shoes and barefoot. Surprisingly, loads, particularly at the knee, were reduced dramatically in the barefoot condition. They weren't able to explain the results, but its likely that a variety of factors, both biomechanical and propioceptive, combine to make unshod walking more efficient and less jarring.