my 2bits..

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I usually simmer until the the smaller bones actually crumble apart in my hands.
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Having done this professionally for years I agree.

The vinegar is not that important as long as you start by adding tomato paste (acidic) during the roasting process.

A small amount of salt also helps, here is what we did for years to make non MSG broth.

-Beef and Veal bones coated with salt and tomato paste in the oven to roast. The tomato paste must roast at the same time as the bones..

-Add to pot and fill with cold water then start the simmering process. Minimum is two days, it can take up to three to get the bones to crumble in your hand.

-About 8 hour before you are finished add your other aromatics like onions, celery etc. good to roast those first as well but that depends on how dark you want the end product.

-Two hour before you are done add your Herbs and if you want to fortify it your ground up meat.

Some Japanese Chefs added seaweed as a Glutamine source to speed things up but IMO you loose something that way..