Add the soy sauce, vinegar, garlic, onion, peppercorns, and bay leaves to your slow cooker.
Slice the chicken into 1 to 1½ inch pieces, add to the slow cooker and mix to coat all the chicken. (This photo shows I used the Variant and went with skinless, bone in, fat trimmed, whole thighs.)
Cover and turn on your slow cooker to the Low setting for 5 to 6 hours. Check for tenderness and doneness at the 5 hour mark.
To thicken the sauce slightly, 30 minutes before serving, set the slow cooker to the High setting, stir, and leave the cover tilted to allow some of the liquid to evaporate.
Remove the bay leaves, serve with rice.