Blend onion, garlic, cinnamon, tomato sauce, 1 tablespoon vinegar, 1 tablespoon brown sugar, salt and pepper in a bowl. Place ribs in slow-cooker bowl and pour sauce over top. Cover and cook on LOW for 9 hours.
Transfer ribs to a platter. Skim and discard excess fat from liquid. Stir in remaining 2 teaspoons vinegar and 1 teaspoon brown sugar. Top ribs with sauce. Serve with cooked noodles, if desired.