Combine olive oil, balsamic vinegar, minced garlic, Italian seasoning, salt, black pepper, and red pepper flakes in a small bowl. Whisk until the marinade emulsifies, creating a glossy mixture that coats vegetables evenly.
Arrange all prepared vegetables in a large shallow dish or disposable aluminum pan. Pour the marinade over the vegetables, tossing gently with tongs to ensure every piece gets coated with the garlic-herb mixture.
Heat your grill to medium-high heat (around 400°F) for 5-10 minutes before cooking. This ensures proper searing and prevents sticking while the crowd-pleasing grilled veggies develop beautiful char marks.
Working in batches to avoid overcrowding, place marinated vegetables directly on grill grates. Grill for 3-4 minutes per side, watching for caramelized edges and light char marks developing across the surface.
Transfer grilled vegetables to a clean serving platter as each batch finishes cooking. Return the sizzling grilled veggies crowd to the grill if needed for perfect caramelization on all sides.
Drizzle any remaining marinade over the finished vegetables, then scatter fresh basil over the top. Taste and adjust seasonings if necessary before serving to your eager guests.
Serve immediately while the vegetables maintain their warmth and textural appeal. This vegetable medley stays delicious for up to one hour after grilling if covered loosely with foil.