Pierce chicken with fork.
Place thyme, rosemary, vinegar and olive oil in a sealable plastic bag.
Knead to mix.
Place chicken in the bag and knead to distribute marinade.
Marinate in fridge.
Preheat grill.
Remove thyme and rosemary sprigs from bag and place on coals.
Brush grill rack with vegetable oil.
Grill for 5 minutes on each side or until juices run clear when pierced.