Cover 4 dried ancho peppers with water. Let soak for 20 minutes. Drain the peppers reserving the water. Slit peppers open and discard the stems and seeds. Put the pepper and 1/4 cup of soaking liquid in a food processor or blender. Add vinegar, garlic, salt, cumin, oregaon, pepper, ground coriander, and cinnamon. Process to a smooth paste. Pour paste into a large bowl or zip lock bag and add the cubed pork. Toss to coat meat with marinade. Refrierate for at least 12 hours turning occasioinally.
Heat in a large, heavy dutch oven over medium heat, add vegetable oil and onions. Cover and cook, stirring often, until onions are softened but not browned, about 5 minutes. Add the pork, marinade and tomatoes. Reduce heat, cover the pan and simmer until tender, 1 1/2 to 2 hours. Remove meat to a bowl and keep warm. Spoon fat off sauce then boil the sauce over high heat until a bit thickened. Serve over hot rice or warmed flour or corn tortillas.