Place the knuckle and marrow bones and optional calves foot in a very large pot with vinegar and cover with water.
Let stand for one hour.
Meanwhile, place the meaty bones in a roasting pan and brown at 350 degrees in the oven. When well browned, add to the pot along with the vegetables.
Pour the fat out of the roasting pan, add cold water to the pan, set over a high flame and bring to a boil, stirring with a wooden spoon to loosen up coagulated juices.
Add this liquid to the pot.
Add additional water, if necessary, to cover the bones; but the liquid should come no higher than within one inch of the rim of the pot, as the volume expands slightly during cooking.
Bring to a boil.
A large amount of scum will come to the top, and it is important to remove this with a spoon.
After you have skimmed, reduce heat and add the thyme and crushed peppercorns.
Simmer stock for at least 12 and as long as 72 hours.
Just before finishing, add the parsley and simmer another 10 minutes.
You will now have a pot of rather repulsive-looking brown liquid containing globs of gelatinous and fatty material. (It doesn’t even smell particularly good. But don’t despair. After straining, you will have a delicious and nourishing clear broth)
Remove bones with tongs or a slotted spoon. Strain the stock into a large bowl.
Let cool in the refrigerator and remove the congealed fat that rises to the top.
Transfer to smaller containers and to the freezer for long-term storage