Combine first 4 ingredients, 1/4 teaspoon black pepper, 1 tablespoon oil, and 1/4 teaspoon salt. Place bell peppers, skin sides down, and onion on grill rack coated with cooking spray; grill 10 minutes. Turn onion; add eggplant to grill. Remove bell peppers.
Place bell peppers in a zip-top bag; seal. Let stand 10 minutes. Grill eggplant and onion 5 minutes; remove onion. Turn eggplant; grill 5 minutes. Remove eggplant. Add tomatoes to a grill basket; grill 5 minutes.
Remove bell peppers from bag. Peel and discard skins; slice lengthwise.
Combine 1/4 teaspoon salt and vinegar. Slowly add 2 tablespoons oil, stirring with a whisk.
Combine vegetables, dressing, garlic, and olives. Sprinkle with 1/4 teaspoon salt, 1/4 teaspoon pepper, basil, and chives.