Make the dressing by whisking together 3 tablespoons oil with the vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. Set aside.
Heat a large skillet over high heat for 5 minutes. Add 2 tablespoons oil and heat until it starts to smoke. Add the corn, zucchini, bell pepper, and onion and cook, stirring occasionally, until the vegetables are crisp-tender and starting to brown, about 5 minutes. Add the garlic and cook, stirring, for 30 seconds. Add the dressing, stir to combine, then remove the skillet from the heat. Transfer the vegetables to a bowl and let cool.
Once the vegetables have cooled to room temperature, add the tomatoes and basil. Stir to combine, and season to taste with additional salt and pepper, if needed. Can be served at room temperature or cold. If serving cold, cover with plastic wrap and refrigerate.