Preheat the grill to medium-high heat. In a large bowl, combine the zucchini, bell peppers, red onion, and cherry tomatoes. Drizzle with 3 tablespoons of olive oil, then sprinkle with salt, black pepper, and dried oregano. Toss to coat the vegetables evenly.
Place the vegetables on the grill. Grill for about 4-5 minutes per side, or until they are tender and have nice grill marks. Remove from the grill and let cool slightly.
While the vegetables are grilling, prepare the lemon vinaigrette. In a small bowl, whisk together 1/4 cup olive oil, lemon juice, Dijon mustard, minced garlic, and salt and pepper to taste.
Once the grilled vegetables have cooled, chop them into bite-sized pieces and place them in a large serving bowl. Drizzle the lemon vinaigrette over the vegetables and toss to combine.
Top the salad with crumbled feta cheese and fresh basil. Serve immediately or chill in the refrigerator for 30 minutes before serving for enhanced flavors.
