In a very hot pan, cook each of the vegetables seperately over a medium-high flame with as little of the olive oil as possible. Cook the onions and the garlic together in the same pan in a little more oil. Set all cooked vegetables aside in a large bowl or pot.
Make the "agrodolce": Combine vinegar and sugar in a small saucepan and reduce by half over medium-high heat for 3 to 4 minutes.
While vegetables are still warm, combine with the roasted peppers, olives, and capers. Pour the "agrodolce" vinegar over the vegetable mixture and toss to coat. Add salt and pepper to taste. Add the chopped herbs, marinara sauce and diced tomatoes and stir.