Prepare the adobo sauce: In a large pot over medium-high heat, combine vinegar, soy sauce, water, brown sugar, peppercorns, quartered onion, whole garlic cloves, bay leaves, cinnamon stick, and thyme sprigs.
Add chicken: Place the chicken thighs into the pot. Don't worry if they're not fully submerged in the liquid - this is normal.
Simmer: Reduce heat to maintain a gentle simmer. Cover the pot and cook for 60-75 minutes, occasionally checking to ensure the liquid hasn't reduced too much.
Check for doneness: The chicken is ready when it falls apart easily when prodded with a fork. If it's not quite tender enough, continue cooking for another 10-15 minutes.
Shred the chicken: Using two forks, shred the chicken directly in the pot, allowing it to absorb more of the flavorful cooking liquid.
Remove aromatics: Fish out and discard the thyme stems, bay leaves, cinnamon stick, and as many peppercorns as you can find.
Warm tortillas: Heat your corn tortillas in a dry skillet or directly over a gas flame until slightly charred and pliable.
Assemble tacos: Fill each tortilla with a generous spoonful of the shredded adobo chicken. Top with chopped cilantro, diced onion, and a dollop of sour cream. Serve with lime wedges for squeezing over the top.