In a 5- to 6-quart slow cooker, combine the onions, garlic, vinegar, soy sauce, brown sugar, bay leaf, and ¼ teaspoon pepper. Place the chicken on top and sprinkle with the paprika.
Cook, covered, until the chicken and onions are tender, on low for 7 to 8 hours or on high for 4 to 5 hours (this will shorten total cooking time).
Twenty minutes before serving, cook the rice according to the package directions.
Ten minutes before serving, if the slow cooker is on the low setting, turn it to high. Gently fold the bok choy into the chicken and cook, covered, until tender, 3 to 5 minutes. Serve with the rice and sprinkle with the scallions.