Soak the cedar planks in water for at least 1 hour.
In a deep saucepan, combine the soy sauce, vinegar, brown sugar, and garlic. Bring to a boil then lower the heat and gently simmer, stirring occasionally, until reduced by half and thick, about 15 minutes. Remove from the heat, stir in the honey, sesame oil, ginger, and red pepper flakes. Let cool for a few minutes. Reserve ½ cup for later.
Put the chicken thighs in a mixing bowl or baking pan and cover with the marinade, making sure the thighs are submerged as much as possible. Refrigerate until time to grill.
Prepare grill for direct and indirect heat and preheat to medium (about 400 degrees).
Place the cedar planks over direct heat, close the lid, and grill them for 10 minutes or until they start to smoke.
Flip them over and place on the indirect side of the grill. Put 4 thighs on each plank. Discard the marinade. Cover the grill and cook for 15 minutes. Glaze with the reserved marinade and cook for another 15 minutes or until they reach an internal temperature of 165 degrees, glazing every few minutes.