Place casing in medium bowl. Cover generously with cold water; stir in vinegar. Allow casing to soak 30 minutes.
Drain casing; fit one end of casing over mouth of faucet. Holding casing in place with hand, begin running a thin stream of cold water slowly through length of casing, untwisting as water passes through. Increase water flow slightly to stretch casing to full width. Check for holes in casing that allow water to escape, indicating damaged spots. To remove any damaged spot, cut off the casing above and below it; you will then be working with several shorter pieces instead of one entire length. Allow water to run through casing or casing pieces 2 minutes. Return casing to bowl of cold water.
Pass veal through meat grinder fitted with fine disc into plate or bowl; grind pork fat into same bowl. Sprinkle onions, parsley, lemon zest, salt, and pepper over meat mixture; stir with wooden spoon until evenly distributed. Pass mixture through grinder at least twice. Beat mixture vigorously with wooden spoon until creamy and light.
Remove blade and disc from grinder; attach sausage funnel. Drain sausage casing thoroughly; run fingers gently down length of casing, pressing thumb and forefinger together, to remove water. Pass forcemeat through grinder until even with end of funnel in order to prevent an initial air pocket. Fit one end of casing over mouth of funnel. Pull casing up onto funnel, leaving 4 inches hanging. Tie unattached end of casing into double knot to seal.
Pass veal mixture through funnel into casing, easing casing off funnel gradually as casing fills out. Work at steady pace, making sure casing is evenly and firmly packed. If any air pockets form in casing, pierce them with needle. Continue filling casing until all veal mixture is used. Tie top end of casing into double knot. To form smaller sausages, tie casing tightly at 4-inch intervals with short pieces of butcher's twine.
Heat large kettle or stockpot of salted water over medium heat to boiling. Reduce heat to lowest possible setting. Form sausages into coil and place in large flat plate. Slip sausages into water; cook, covered, until firm, about 20 minutes. Remove carefully with slotted skimmer to warmed serving platter; spoon some of the cooking liquid over.
Directions in this recipe are based on the use of a hand-operated meat grinder and its sausage funnel attachment. If using an electric meat grinder and attachment, follow the manufacturer's directions.