Swordfish with Vegetable Couscous and Tomato Vinaigrette

1 1/2 teaspoons wine vinegar

3 tablespoons olive oil

1/2 teaspoon dried oregano

Fresh-ground black pepper

1 3/4 teaspoons salt

2 scallion s including green tops, chopped

1 fennel bulb, cut into 1/4-inch dice

1 summer squash , cut into 1/4-inch dice

2 1/2 cups canned low-sodium chicken broth or homemade stock

11/3 cups couscous

4 swordfish steaks (about 2 pounds in all)