Preheat grill to medium.
Combine tomatoes, squash, onion, green beans, olives, lemon juice, oregano, oil, capers, 1/4 tsp salt and 1/4 tsp pepper in a large bowl.
To make a packet, lay two 20-inch sheets of foil on top of each other, coat the top piece with a layer of avocado oil. Place one portion of tilapia in the center of the foil. Sprinkle with some of the remaining 1/4 tsp salt and pepper, then top with about 3/4 cup of the vegetable mixture. Bring the short ends of the foil together, leaving enough room in the packet for steam to gather and cook the food. Fold the foil over and pinch to seal. Pinch seams together along the sides. Make sure all the seams are tightly sealed to keep steam from escaping. Repeat with more foil, cooking spray and the remaining fish, salt, pepper and vegetables.
Grill the packets until the fish is cooked through and the vegetables are just tender, about 5 minutes.