Lay the chicken thighs in the slow cooker with the fleshier side facing down. In a small bowl, whisk together the soy sauce, fish sauce, rice vinegar, black pepper, garlic, ginger, canola oil, and brown sugar. Pour over the chicken, put on the lid, and cook until the chicken is very tender, 5 to 6 hours on low or 2 to 3 hours on high. The time may vary depending on the size of the chicken thighs.
When the chicken is done, cut the heads of baby bok choy lengthwise into 6 wedges and wash.
About 10 minutes before the cooking time is up, remove the chicken from the pot and add the bok choy, nestling into the sauce. Put the chicken on top of the bok choy, shut the lid, and cook until the bok choy is tender (about 8 minutes on high). Serve over rice with sriracha on the side for those who want to add a kick of heat.