Remove stems and most seeds from chiles. In dry skillet over medium-high heat, toast chiles just until fragrant, stirring often, approx. 2 min. Add chicken stock and a few dashes hot sauce, if desired, and remove from heat; let steep 15 minutes.
In a Dutch oven or instant cooker, heat oil to sauté (high); cut meat into 1 lb chunks, season generously with salt and pepper and sear on all sides. Remove meat from pot and set aside. To the pot, add onion, cooking until soft, approx. 2 min. Add garlic and spices and cook 1 min. Add vinegar and tomatoes, bringing to a simmer.
Spoon about half of mixture into chilies, transferring chili mixture to a blender or processor and puree until smooth (hold lid down or steam will explosively pop the top). Add meat to pot, covering with pureed chili sauce. Add bay leaves, cover tightly and cook until very tender (3 hours in 325F oven, or 45 min. in instant cooker on high). Shred and remove meat, reserving all sauce.
To assemble tacos, dip tortillas in fat layer on surface of sauce, and place in well-oiled nonstick or cast iron skillet. Sprinkle a tablespoon of cheese and two tablespoons of meat on each tortilla, folding over when cheese melts. Continue frying on each side until crispy. Serve immediately, garnished with limes, cilantro, onion, and plenty of warm birria sauce for dipping.