Build a wood fire* in a camp grill or fire ring, using about 4 logs and some kindling; let burn to medium (you can hold your hand 5 in. above cooking grate only 5 to 7 seconds), about 1 hour. Adjust fire so there's a thick area of embers and smaller logs in the middle and larger logs to the sides.
Smash garlic, put in a small bowl with vinegar, and set aside. In a large bowl, toss zucchini, corn, and tomatoes with 2 tbsp. oil, 1/2 tsp. salt, and 1/4 tsp. pepper.
Place onions in embers between some smaller logs and cook, turning every 10 minutes or so, until completely black and soft when squeezed with tongs, 25 to 40 minutes. Meanwhile, set peppers on embers and cook, turning every few minutes, until completely charred, about 20 minutes. Transfer vegetables to a board and let cool.
Set cooking grate in place, if using a portable one. Grill zucchini, corn, and tomatoes (in batches, if needed), turning occasionally, until grill marks appear, 5 to 35 minutes, depending on distance from fire.
Pull off blackened outsides from onions and peppers. Cut corn kernels from cobs into large bowl. Cut remaining vegetables into slices or strips, discarding seeds; add to bowl.
Stir remaining 6 tbsp. oil into vinegar with remaining 1/4 tsp. each salt and pepper. Toss gently with vegetables, add mint, and more salt and pepper if you like.
*Or cook all the vegetables over (but not in) a medium (350° to 450°) charcoal fire, adding 8 briquets every 30 minutes.
Note: Nutritional analysis is per 1 1/2-cup serving.