Place the pork butt pieces into a large pot or Dutch oven and 1 tsp of salt. Pour enough water over the pork for the liquid to reach the top of pork cubes (but not cover it), about 3 to 4 cups.
Bring the pot to a boil on high, remove any foam . Then turn the heat down to low and simmer uncovered for 1 1/2-2 hours or until the liquid is almost gone and the meat is fork tender.
In the same pot, add 3 tbsp of vegetable oil or lard. Cook the pork cubes, and fry for 5 minutes on medium-high Remove from heat and set aside..
Meanwhile, place the guajillo chiles, bay leaves, and garlic in a medium pot, cover with water and bring to a boil. Simmer over medium heat for 10 to 12 minutes until the chiles are completely rehydrated and plumped up.
Place rehydrated guajillos, garlic, and bay leaves in the blender, along with 1 cup of the cooking liquid, the oregano, coriander seeds, remaining 1/2 teaspoon salt, black pepper, cumin, and vinegar. Puree until completely smooth.
Pour the chile puree onto the meat, mix well, and, cook, partially covered, for another 10 to 15 minutes. Stir vigorously, but only occasionally. By the end, the meat should be so tender and luscious that it falls apart completely, breaking into shredded pieces. Turn off the heat.