Cut a deep horizontal pocket in the side of each chicken breast. Make the pocket as large as you can without piercing the top or bottom of the breast. Place 2 slices of tomato and about 4 basil leaves in the pocket of each chicken breast. Secure the pocket with toothpicks threading along the side to close. Heat the oil in a heavy oven-proof skillet until it begins to smoke. Cook each side of the chicken until golden brown. Add the vinegar and chicken stock and bring to a boil. Lower the heat and gently simmer the chicken for 2 or 3 minutes per side until cooked through. Remove the chicken breasts from the skillet and keep warm. Continue to cook the sauce until it is reduced to a thick syrup. Taste the sauce and and season with salt and pepper. Spoon the sauce over each chicken breast.