1. Combine the vinegar with the dried oregano in a small glass jar. Let steep at room temperature for at least 2 days and up to 2 weeks. When ready to use, stir in the olive oil. Set the spatchcocked chicken, skin side down, in an enameled or glass baking pan to catch the liquid. Drizzle the vinegar mixture over the chicken and rub it in. Allow to marinate 1–2 hours at room temperature or up to 6 hours in the refrigerator (if refrigerated, let sit out at room temperature for 1 hour before cooking).
2. Mix together sage, rosemary, black pepper, salt and red pepper flakes in a small bowl. Rub the mixture together with your fingers to lightly crush the herbs and release their oils. Remove the chicken from the pan and reserve the marinade for basting. Pat the chicken dry. Rub the herbs all over both sides of the chicken.
3. Prepare a grill (preferably charcoal or wood) for indirect cooking over medium heat (350–400F). Scrub the grates clean and lightly oil with grapeseed oil. Place the chicken, skin-side down, on the cool side of the grill and cook with the lid on and the top vent half open until the skin releases easily from the grates, 15–20 minutes. Turn chicken over and baste with the reserved marinade. Continue to cook with the cover on 30–45 minutes more, basting with the marinade every 15–20 minutes, until a thermometer inserted in the thickest part of the breast registers at 140 and the thighs register 155.
4. Transfer chicken to a cutting board and tent with foil. Allow to rest for at least 15 minutes before carving to allow the meat time to reabsorb the juices (discard any remaining marinade). Cut the hindquarters from the chicken and separate the legs and thighs. Cut the two breast halves off the bone and cut each in half. Arrange the chicken on a platter, pour any accumulated juices over the top and serve.
Note: To spatchcock a chicken, place it, breast-side down, on a cutting board and use poultry shears to cut along each side of the backbone to remove it. Turn the chicken over and press firmly on the breastbone with the heel of your hand until it cracks and allows the bird to slightly flatten. Tuck the wing tips behind the breast or remove completely to keep them from charring.