1/2 cup tarragon vinegar
1/2 cup sugar
1/2 cup vegetable oil plus additional for brushing
2 tablespoons Dijon mustard
1 tablespoon chopped fresh tarragon
1 medium head of red cabbage (about 1 3/4 pounds), quartered through core
1 medium head of green cabbage (about 13/4 pounds), quartered through core