Place chicken thighs, peppercorns, garlic, bay leaves, soy sauce, and vinegar into the bottom of a slow cooker in that order, making sure peppercorns and bay leaves are submerged into the liquid.
Cover and cook on High until chicken thighs are no longer pink in the centers, about 5 hours. An instant-read thermometer inserted near the bone of a thigh should read 165 degrees F (74 degrees C).