Place each chicken breast between 2 pieces of plastic wrap. Using the flat side of a meat mallet, pound lightly to 1/2-inch thickness. Remove plastic wrap.
Place chicken in a self-sealing plastic bag. In a small bowl combine vinegar, oil, garlic, kosher salt, and crushed red pepper. Pour over chicken. Seal bag; turn to coat chicken. Cover and marinate in the refrigerator for 1 to 4 hours, turning bag occasionally.
Drain, discarding marinade. For a charcoal grill, grill chicken on the rack of an uncovered grill directly over medium coals for 10 to 12 minutes or until chicken is no longer pink (170°F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place chicken on grill rack over heat. Cover and grill as above.)
Oven method: Place chicken on a rack in a shallow roasting pan. Roast chicken in a 400°F oven for about 15 minutes or until chicken is no longer pink (170°F).