Add the remaining 3 tablespoons of butter and the garlic to the skillet drippings. Sauté the garlic for 30 seconds, being careful not to burn it. Add the chicken broth and capers to the skillet, stirring to mix. Cook the liquid for 3 to 5 minutes over medium-low heat, until reduced by half. Add the lemon juice and vinegar, and heat through. Return the chicken to the skillet, spooning some of the sauce over the chicken. Cover the skillet and cook for an additional 8 to 10 minutes over medium heat, until the sauce bubbles and the chicken is cooked through. Sprinkle with chopped parsley and serve.