Cut each chicken breast half into two pieces, crosswise. Place chicken pieces between two sheets of heavy-duty plastic wrap, and pound to ¼-inch thickness with a meat mallet or a rolling pin.
Heat oil in large skillet over medium-high heat. Meanwhile, combine flour, salt, and pepper in a shallow bowl. Dredge chicken in flour mixture, shaking to remove excess. Saute chicken in skillet 2 to 3 minutes on each side, or until chicken is no longer pink; transfer to warmed plates.
Pour off any excess oil. Add vermouth to skillet and cook over medium-high heat 2 minutes, or until mixture is reduced to ¼ cup, stirring to loosen particles from bottom of skillet. Stir in lemon juice, butter, and capers. Spoon sauce over chicken; sprinkle with lemon zest, if desired, and parsley. Garnish with lemon wedges and serve immediately.