Brush eggplant, squash, and bell peppers evenly with 4 teaspoons oil. Sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon black pepper. Place vegetables on grill rack; grill 3 minutes on each side or until slightly charred. Cool and chop. Place vegetables in a large bowl.
Bring 2 cups water and broth to a boil in a medium saucepan. Stir in couscous. Reduce heat, and simmer 8 minutes or until couscous is tender. Drain and rinse with cold water. Add couscous, remaining 3/4 teaspoon salt, and remaining 1/4 teaspoon pepper to vegetable mixture; toss. Combine vinegar and mustard in a medium bowl, stirring well. Gradually add remaining 1/4 cup oil to vinegar mixture, stirring constantly with a whisk. Drizzle vinegar mixture over couscous mixture; toss to coat. Stir in basil.