Put the marrow bones and knuckle, as well as the calf's foot (if you are using it) in a big pot with the vinegar and cover with cold water. Let them stand for an hour.
Meanwhile, roast the meaty bones at 350 degrees F in the oven.
When they are nice and brown, add them to the pot with the marrow bones and knuckle.
Add the vegetables to the pot too. Pour the fat out of the roasting pan, add cold water to the pan and heat it over a high flame, poking at the stuck on bits with a wooden spoon until they come loose.
Tip all this liquid into the pot, adding more water if you need to, to cover the bones. The liquid should not be any higher than an inch from the top of the pot.
Bring the beef broth to a boil and remove the scum from the top with a spoon.
Turn down the heat and add the peppercorns and thyme.
Boil the broth for between 12 and 72 hours.
Add the parsley and simmer for another 10 minutes.
Remove the bones with a slotted spoon or tongs and strain the beef broth into a big bowl.
Let it cool in the refrigerator, and then remove the congealed fat, which will rise to the top.
Transfer the beef broth to smaller containers, use what you need and freeze the remainder.