Transfer the dried chili peppers to a saucepan and fill with water. Bring the water to a full boil. Turn off the heat and allow the chilies to soak for at least 10 minutes.
While the chilis are soaking, toast the spices. Place the black peppercorns, coriander seed, bay leaf, and cinnamon stick in a small skillet and heat over medium heat. Allow the spices to toast for 3-4 minutes, stirring or shaking the skillet frequently to avoid burning. Wrap the spices in cheesecloth and bind the spice sachet with twine.
Once softened, use a slotted spoon to remove the chiles from the hot water. Allow them to cool enough to handle, then remove the stems and seeds. Transfer the chilies to a high-powered blender along with the chopped onion, garlic, and chicken stock. Blend until smooth.
Place the boneless chicken thighs in the bottom of your crock pot. Pour the birria sauce through a fine mesh strainer to stain out the pulp. Press on the pulp with the back of a wooden spoon to extract as much liquid as possible.
Add the spice sachet to the crock pot along with the cider vinegar, tomato paste, dried oregano, ground cumin, and sea salt. Give everything a big stir.
Secure the lid on the crock pot and cook on Low heat for 8 hours (recommended) or High heat for 4 hours.
Once the slow cooking process has finished, use tongs to transfer the cooked chicken to a cutting board. Use two forks to shred the chicken. Place the chicken back into the crock pot with the sauce.