Preheat your grill to medium-high heat and lightly oil the grates. Toss the zucchini, yellow squash, bell peppers, red onion, cherry tomatoes, and corn with olive oil, salt, and black pepper. Grill the vegetables until tender and slightly charred, about 3-5 minutes per side. Remove from the grill and let cool slightly. Cut the corn off the cobs and chop the grilled vegetables into bite-sized pieces. In a small bowl, whisk together olive oil, balsamic vinegar, Dijon mustard, garlic, salt, and pepper to make the dressing. Toss the grilled vegetables with the dressing in a large bowl. Garnish with crumbled feta cheese and fresh basil before serving.