Mix the breadcrumbs, milk, parsley, egg, Worcestershire sauce, onion, chili powder and garlic in a large bowl. Add the meat, season with salt and pepper and mix with your hands until combined. Shape into six 3-to-4-inch oval loaves.
Heat the vegetable oil in a large nonstick skillet over high heat. Add the loaves and brown about 3 minutes per side.
Whisk the ketchup, sugar and vinegar in a bowl and brush a few tablespoonfuls over the meat. Add 1/2 cup water to the skillet, cover and simmer over low heat until cooked through, about 15 minutes.
Transfer the loaves to a plate. Add the remaining ketchup mixture to the skillet and cook over high heat, stirring, until thick, 3 to 5 minutes. Serve 4 meatloaves with the hot glaze, reserving 2 for quesadillas.