1. Whisk together 1/3 cup vinegar, the rosemary, and 3 tablespoons garlic in a small bowl. Add 1/2 cup oil in a slow stream, whisking until thickened and emulsified; salt and pepper to taste, for marinade. Divide chicken and marinade between 2 large heavy-duty resealable plastic bags and seal. Turn until chicken is coated well. Marinate, chilled, at least 2 hours or overnight, placing bags in large bowls to catch any leaks.
2. Whisk together remaining 2 tablespoons vinegar, 1 tablespoon garlic, and the shallot in a serving bowl. Add remaining 3 tablespoons oil in a slow stream, whisking, until thickened and emulsified; salt and pepper to taste, for dressing. Add tomatoes, cilantro, and basil to dressing; toss gently to coat.
3. Preheat grill and lightly oil rack. Remove chicken from marinade (discard marinade) and grill on rack over moderate heat, turning once, until cooked through, about 35 minutes. Transfer to a serving platter. Halve sausages lengthwise and grill on lightly oiled rack over moderate heat, turning, until heated through, about 6 minutes. Transfer to platter with chicken. Top all with herbed tomato salad and serve remaining salad on the side. Makes 16 servings.