Clean and trim the excess fat and tissue from skirt steak. If needed, cut the skirt steak to a size that will lie flat on the grill or the cast-iron skillet.
In a food processor, combine vinegar, oil, shallots, garlic, cilantro, parsley, oregano, crushed pepper flakes and sugar. Once the marinade is blended, it should be a little thick and very green. Place the marinade in a bowl with the steak and let marinate overnight in the refrigerator.
Heat the grill or the cast-iron skillet to high. Remove the meat from the refrigerator to let it come to room temperature, about 20 minutes.
Place the meat on a hot grill or cast-iron skillet for about 2-4 minutes to allow grill marks to be made before turning or flipping. After both sides have grill marks on them, remove the meat from heat and place on a cutting board. Cut steak into 2-inch thick pieces, then turn and cut against the grain. Sprinkle with salt. Serve.