Preheat the oven to 450 degrees. Spray a pan with nonstick cooking spray.
Combine the brown sugar, chili powder, oregano, cumin, vinegar and 1/4 teaspoon salt in a small bowl. Rub all over the pork.
Put the pork in the pan and then into the oven for about 25 minutes.
Meanwhile, heat a large skillet over medium low heat. Swirl in the oil, then add the onions, sugar, the salt and the water. Cook uncovered, stirring occasionally, until softened and lightly browned, about 20 minutes.
Stir in the tomato sauce and simmer, uncovered for about 5 minutes so the flavor blends. Remove from heat.
Let the pork stand 10 minutes after removing from heat. While the pork rests, wrap the tortillas in foil and warm in the oven for about 10 minutes.
Slice the pork and stir into the onion mixture. Spoon about 1/2 a cup of the meat mixture onto each tortilla and fold up to eat.