1. Preheat grill to medium heat.
2. Add red bell pepper, yellow bell pepper, zucchini, yellow squash, red onion, olive oil, garlic, salt, and ground pepper to large bowl. Toss to combine.
3. Place vegetables on oiled grill rack. Grill 10 minutes, turning once.
4. In small saucepan, bring vinegar, sugar, and soy sauce to a boil.
5. Reduce heat to low, simmer 5 minutes or until mixture thickens.
6. Pour sauce over vegetables.