In a small bowl, mix vinegar, olive oil, cilantro, garlic, cumin seed, 3/4 teaspoon salt, and chilies to taste.
Rinse fish and pat dry. In a heavy zip-lock plastic bag (1-gal. size), combine fish and 1/3 cup of the vinegar mixture. Let stand, turning occasionally, for about 15 minutes.
Meanwhile, rinse and drain cabbage. Cut through core into quarters; cut out and discard core sections. Slice each quarter lengthwise into thin shreds to make about 6 cups. In a large bowl, mix shredded cabbage, onion, and 1/2 cup of the vinegar mixture. Add salt to taste.
Lift fish from marinade (discard marinade) and place on an oiled grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook fish, turning once, until barely opaque but still moist-looking in center of thickest part (cut to test), 6 to 8 minutes.
Mound cilantro-chili slaw equally on four dinner plates. Top each mound with a piece of hot or warm grilled fish. Drizzle remaining dressing equally over fish. Add salt to taste.