Heat gas or charcoal grill. In small bowl, mix 2 tablespoons of the oil, the vinegar, chopped basil, mustard, garlic, 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper; set aside.
Drizzle remaining 2 tablespoons oil over yellow squash and zucchini; sprinkle with remaining 1/4 teaspoon each salt and pepper. Place vegetables on grill over medium-high heat. Cover grill; cook 3 to 4 minutes, turning once, until vegetables are tender.
Place vegetables in serving bowl. Drizzle with reserved vinaigrette. Garnish with basil leaves.