How to Make It
Prepare a grill to medium-high heat.
Brush fish sauce evenly over steak; rub steak evenly with sugar and 2 garlic cloves. Let stand while grill preheats.
Brush eggplant, onion, zucchini, and bell pepper evenly with 2 1/2 tablespoons oil. Arrange steak and vegetables on oiled grill rack. Grill eggplant until tender, 2 to 3 minutes on each side. Grill peppers and zucchini until crisp-tender, about 3 minutes on each side. Grill onions until crisp-tender, about 5 minutes on each side. Grill steak until medium-rare (130°F to 135°F) or desired degree of doneness, about 5 minutes on each side. Let steak stand 5 minutes before slicing.
While steak stands, combine remaining 2 1/2 tablespoons oil, vinegar, parsley, oregano, red pepper (if desired), 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Stir well with a whisk or shake together in a jar.
Cut steak diagonally across the grain into slices. Arrange steak and vegetables on a platter; sprinkle evenly with remaining 1/2 teaspoon salt and 1/2 teaspoon black pepper. Serve with vinaigrette.