Combine onions, carrots, green peppers, bay leaves, red pepper, salt, and a few grindings of black pepper in a 3- to 4-quart enameled or stainless- steel saucepan. Pour in the vinegar, 2 tablespoons of the oil and the water, and bring to a boil over high heat. Reduce the heat to low, cover tightly, and simmer for 20 minutes, or until the vegetables are tender but not falling apart.
Meanwhile, pour the remaining 4 tablespoons of oil into a heavy 10- to 12- inch skillet and heat over moderate heat until a light haze forms above it.
Add a few fillets at a time, and cook them for 2 or 3 minutes on each side, or until delicately brown. As they brown, transfer the fillets to a large, shallow heatproof serving dish.
Pour the hot vinegar mixture over the fish, arranging the vegetables evenly and attractively on top. Serve immediately or cool to room temperature, cover tightly, and refrigerate until thoroughly chilled.