A quick google search of “how to make bone broth” will bring up oodles of results. Most will tell you to add a tablespoon or two of apple cider vinegar. In fact, vinegar is always an ingredient in the standard cookbooks, and well-respected nutritionist who recommend bone broth say that you should include vinegar.
The low pH of vinegar is supposed to soften the bones and help leach out the minerals. The idea follows traditional cooking methods, which often smart advice. And even traditional lore about food and health can often be right: for example, chicken broth really does help relieve the symptoms of the common cold.