Combine the bread, egg, and milk in a mixing bowl, stirring to combine, and let rest for 15 minutes. Mash the bread mixture with a fork to make a smooth paste. Add the remaining ingredients and stir to combine. Form the meat into balls about the size of golf balls (or larger if you like) and place them on a greased baking sheet. Bake in a preheated 400 F oven until browned, about 20 minutes.Meanwhile, make the sauce. Heat the oil in a pot over moderate heat and sauté the onion until tender but not brown, about 5 minutes. Add the garlic, oregano, and red pepper flakes and cook for 2 minutes. Add the tomatoes with their liquid, crushing the tomatoes with your hand as you add them. Add the tomato paste, basil, vinegar, salt, and pepper and bring to a boil. Reduce the heat and simmer uncovered for 20 to 30 minutes. Add the meatballs and simmer for 20 to 30 minutes.To serve, toss about 1 C. of the sauce with the drained spaghetti and divide it between 4 to 6 serving plates. Top with the meatballs and remaining sauce and serve with freshly grated Parmesan cheese.