Hay is an old trick in curing concrete in hot weather. In hot weather, concrete tends to dry on the surface first, then when the rest dries, the surface will develop hairline cracks. (an expansion-contraction thing)
To keep this from happening, after finish troweling, the hay is scattered over the surface and wet down to slow the surface drying. It simply acts as a moisture absorber, and keeps the concrete wet for a much longer time than just water, because the water without the hay would quickly evaporate.
I think in the case of the vinegar soaked hay, it acts the same way.