Brush the corn liberally with olive oil and season well with salt and pepper. Grill, turning every few minutes, until light gold all over and cooked, about 12 minutes. Let cool and cut off the kernels. Discard the cobs.
Core the tomatoes and cut a small X on the bottom of each. Brush with olive oil, season with salt and pepper, and place on the grill, X side down, away from direct heat. Cover the grill and cook until the tomatoes begin to soften but are not cooked all the way through (or they will melt through the grate!), about 15 minutes. Set aside until cool enough to handle, then peel. Cut the tomatoes in half crosswise and squeeze out the juice and the seeds through a sieve into a bowl. Reserve the juices and chop the flesh.
Put the onions in the non-reactive medium bowl and toss with 2 tablespoons of the vinegar. Let marinate until the color changes, about 10 minutes.
Add the chopped tomatoes, reserved tomato juice, onions, basil, and 1/3 cup olive oil to the corn. Toss well. Taste for seasoning and adjust with salt, pepper, and remaining vinegar. The salsa is best eaten the same day but will keep, covered and refrigerated, a day or so.
Serve with tortilla chips or as a topping for tacos.