In a medium-sized bowl, whisk together balsamic vinegar, lemon juice, olive oil, Dijon, brown sugar, garlic, oregano and rosemary. Season with salt and pepper, as per taste. Also reserve 1/4 cup and set it aside.
In a gallon size large bowl, mix balsamic vinegar mixture and chicken; marinate for at least 1 hour to overnight, turning occasionally. Then drain the chicken from the marinade.
Preheat grill to medium-high heat. Add chicken to grill and cook, flipping once and basting with reserved 1/4 cup marinade until cooked through, about 5-6 minutes on each side.
Serve immediately, garnished with parsley, if desired.