1) For the sauce: Scoop the flesh and juices of the tomatoes out of the skins with a spoon into a bowl; discard the empty skins. Whisk in the vinegar, garlic, salt, pepper, and red pepper flakes, breaking up the tomato as you do. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a chunky, slightly thick dressing. Roughly chop the basil and stir into the dressing. Set aside for 15 mins to let the flavours come together. Remove the garlic.
2) For the vegetables: Pour 2.5cm or more of water in a wok or skillet and bring to a boil over medium-high heat. Place the vegetables in a bamboo or collapsible steamer. Set the steamer over the boiling water, cover, and cook for 5 to 8 mins, or just until the vegetables are crisp tender. Serve warm or at room temperature with the dressing.