Place diced chicken thighs in a large container. Combine the Kikkoman Sushi Vinegar, Kikkoman Katsu Sauce, Kikkoman Thai Style Chili Sauce and garlic. Pour sauce mixture over chicken pieces, stir, cover and marinate chicken overnight in mixture.
In a large sauté pan heat 1-2 ounces oil and sauté the shiitake mushrooms until dry and slightly crispy, remove from pan and reserve. Remove chicken from marinade and drain all excess marinade. In the same pan heat the remaining oil and sauté chicken until the internal temperature reaches 165°F. Combine the cooked chicken and shiitake mushrooms.
Just before service sprinkle the chicken and mushrooms with cilantro and scallions. Warm tortillas until softened. Serve 1 ounce of chicken in each corn tortilla.