Heat grill to medium-high heat.
Season the steak with 1/4 teaspoon of the salt and 1/2 teaspoon of the pepper.
Grill to the desired doneness, 4 to 5 minutes per side for medium-rare. Transfer to a cutting board and let rest at least 5 minutes before slicing.
While steak is cooking, sliced eggplant into four 1/2" thick rounds and quarter and seed bell peppers. Brush both sides of the eggplant and peppers with 3 tablespoons of the oil. Season with 1/4 teaspoon salt and 1/2 teaspoon pepper.
Grill until tender, 4 to 5 minutes per side.
Mince garlic cloves and chop parsley. In a small bowl, combine the remaining oil, vinegar, garlic and parsley to make a vinaigrette sauce.
Divide the steak and vegetables onto 4 dinner plates. Spoon sauce over the vegetables or serve on the side.