Grape leaves prevent the fish from sticking to the grill while keeping it tender and moist. Branzino is a variety of sea bass; if you can't find it, you can substitute whole trout.
Heat grill to medium. Very thinly slice zucchini on the bias (preferably on a mandoline). Transfer to a medium bowl, and toss with lemon juice, olives, mint, 2 1/2 teaspoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Let stand while preparing fish.
Score 3 slits in both sides of each fish using a sharp paring knife. Season with some pepper and 1/8 teaspoon salt per side. Arrange 4 sets of 2 grape leaves side by side on a work surface. Place 1 fish on top of each set, and wrap to secure. Brush with remaining 2 teaspoons oil.
Grill fish until cooked through, 4 to 5 minutes per side. During last half of cooking, arrange lemon wedges around fish, and grill until caramelized and softened, 2 to 3 minutes per side. Transfer fish and lemons to a serving plate. Serve wrapped fish with zucchini salad and grilled lemons.