For the chicken: Combine the oyster sauce, garlic, sriracha, sesame oil and black pepper in a large mixing bowl. Add the chicken, toss to coat and massage the mixture into the chicken.
Heat a large stockpot over high heat and add some canola oil. Add the chicken mixture to the pot and saute until the chicken is brown, 5 to 8 minutes. Add the vinegar and soy and stir, then bring to a boil. Reduce the heat to medium/low and place the lid on the pot. Cook and simmer for 35 minutes, then cut the heat. Let the chicken rest for 5 minutes, then pull to make tacos (or serve over rice). Reserve 1/2 cup of the adobo sauce for the bok choy.
For the tacos: Heat the canola oil in a large skillet over medium heat. Saute the bok choy until slightly wilted, about 2 minutes. Add the 1/2 cup reserved adobo sauce and cook for 1 more minute.
Place some of the pulled chicken onto a warm tortilla and top with some bok choy and crumbled chicharrones. Repeat with the remaining tortillas and filling.