Preheat oven to 300°F. Place chicken halves skin side up in roasting pan. Place 3 slices pancetta atop each chicken half. Pour wine and vinegar over chicken. Sprinkle garlic and rosemary over. Drizzle chicken with olive oil. Cover and bake until chicken is cooked through, about 1 hour 45 minutes. Transfer chicken to baking sheet; tent with foil to keep warm.
Preheat broiler. Pour pan juices into small saucepan. Spoon fat from top of pan juices. Boil juices until reduces to 1 cup, about 25 minutes.
Meanwhile, broil chicken until pancetta and chicken are golden brown, about 6 minutes.
Transfer chicken to plates. Pour reduces pan juices over chicken. Garnish with rosemary sprigs, if desired.
