With a pairing knife, core the cabbage. Bring a large pot of water to a boil, add the vinegar, and place the cabbage in the pot, weighed down to keep it submerged. Cook until the leaves soften, about 12 minutes; remove the cabbage, and cool under running water. Carefully remove sixteen whole leaves from the cabbage, saving the remainder for other uses, and set them aside to drain on paper towels. In a medium saucepan, heat 3 tablespoons of the oil over medium heat, add 1/2 cup of the onions, and cook until translucent, about 3 minutes. Stir in the rice, add 1 cup of the chicken stock, and bring to a simmer. Cover, and cook over low heat 7 minutes. Set aside to cool. In a large Dutch oven, heat the remaining olive oil over medium heat. Add the tomatoes and remaining 5 cups stock. Season with 1/2 teaspoon of the salt and the pepper. Let simmer. In a large bowl, combine the ground meats, egg, bread crumbs, reserved rice, parsley, and the remaining 2 tablespoons salt and some pepper, mixing well. Divide the mixture into sixteen equal portions, and roll each into a sausagelike cylinder about 2 1/2 inches long. Working with one cabbage leaf at a time, place a cylinder perpendicular to the stem. Roll the leaf around the stuffing, forming a snug fit, and tuck in both ends of the leaf to enclose the stuffing completely. Repeat with the remaining cabbage leaves and stuffing. Arrange the cabbage rolls side by side in two layers in the casserole with the tomato sauce, gently shake the pan so the sauce spreads evenly, and return to a simmer. Gently stir cabbage rolls from time to time to prevent them from sticking to the bottom of the pan and scorching. Reduce the heat to low, and simmer gently until the cabbage is very tender and the sauce is thick and flavorful, about 1 1/2 hours. Cover partially during the last 30 minutes. To serve, spoon the tomato sauce over the cabbage rolls.