1. Preheat grill to medium-high heat.
2. Place potatoes in a large saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 15 minutes or until tender; drain. Cool slightly. Cut potatoes in half. Combine potatoes and 2 teaspoons oil in a large bowl, and toss well to coat.
3. Brush onion and bell pepper evenly with 1 teaspoon oil. Place potatoes, onion, and bell pepper on a grill rack coated with cooking spray; grill 5 minutes on each side or until tender. Remove vegetables from grill; cool slightly. Cut bell pepper into thin strips. Cut onion slices into quarters.
4. Combine remaining 2 tablespoons olive oil, basil, and remaining ingredients in a large bowl, stirring with a whisk. Add vegetables to bowl; toss to coat.