Put the veal feet, pig's feet, and the thoroughly trimmed pork skin into a large pot, and add enough water to cover by about 2 fingers.
Bring to a boil over low heat, carefully skimming off the foam that forms on top.
When about half the water has boiled away (this takes several hours, up to 4-5 hours of gentle simmering), add the salt and the vegetables, and continue cooking.
As a test, take out a little of the liquid and chill it: if it sets nicely, the stock is ready; if too loose, cook longer.
Strain the whole amount, and let it cool and set completely in a bowl in the fridge.
Thoroughly skim off the solidified fat from the top.
Warm the set stock, but don't let it boil.
Beat 4 egg whites to stiff peaks, and add them spoonful by spoonful, stirring constantly, to the stock, along with the vinegar, so the liquid clarifies nicely.
Bring to a gentle boil, and simmer quietly until the egg whites bind up the floating impurities and the liquid clears (the classic clarification technique, as in consommé-making).
Strain through a clean kitchen cloth (or several layers of cheesecloth), to get a completely crystal-clear aspic.
Keeps in the fridge for about a week, or can be frozen in portions for months.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.