Bones are rich in calcium, an alkaline earth metal. Vinegar on the other hand is an acid, and it ionises the calcium out of the bone (or in a more day-to-day fashion, dissolves it), leaving the bone soft and more easily decomposed. You could then even use the vinegar in food to make it more rich in calcium, if you fancy that. Also, adding a bit of vinegar to food can make uptake of calcium present in the food more easy.