Prepare the barbecue for direct cooking over high heat (180° to 230°C).
Cut a 10mm slice off the top of each tomato. Discard the tops. With a small knife cut around the inside of the fleshy part of the tomato (do not cut through the bottom) to within 10mm of the skin.
With a teaspoon, scoop out the tomato flesh. Discard the juice and seeds to make room for the stuffing. Lightly salt the inside of the tomatoes and place them, cut side down, on a plate lined with paper towel.
To make the stuffing, lightly brush or spray the onion, capsicum, and zucchini with olive oil. Season with salt and pepper to taste. Cook on a hotplate over direct high heat until tender, 4 to 5 minutes on each side for the onions and 3 to 4 minutes on each side for the capsicum and zucchini. Transfer the cooked vegetables to a cutting board and chop them into 10mm pieces. In a bowl combine the cooked vegetables with the cheese, basil, and vinegar to create the vegetable stuffing.
To make the stuffing, lightly brush or spray the onion, capsicum, and zucchini with olive oil. Season with salt and pepper to taste. Cook on a hotplate over high heat until tender, 4 to 5 minutes on each side for the onions and 3 to 4 minutes on each side for the capsicum and zucchini. Transfer the cooked vegetables to a cutting board and chop them into 10mm pieces. In a bowl combine the cooked vegetables with the cheese, basil, and vinegar to create the vegetable stuffing.
Spoon the vegetable stuffing into the tomatoes. Cook the tomatoes using the indirect setting until the cheese is melted and the vegetables are warm, 8 to 12 minutes. Serve immediately.
