In a small bowl whisk together 2 tablespoons oil, vinegar, basil, marjoram and thyme. Season with salt and pepper to taste, then set aside.
Place tomatoes, zucchini, yellow squash, eggplant and bell peppers on a large-rimmed baking sheet. Drizzle remaining ¼ cup of oil over vegetables and season with salt and pepper to taste. Toss to thoroughly coat vegetables in oil and seasoning.
Place vegetable slices on grill and cook until browned on both sides, about 3 minutes per side for zucchini, squash and eggplant, and about 5 minutes per side for tomatoes and peppers. Transfer vegetables to cutting board and cut into large dice.
Transfer diced vegetables to a large bowl, pour in herb vinaigrette and toss to combine. Serve immediately while warm or at room temperature.