Heat a gas grill to medium or prepare a charcoal fire with medium- and low-heat areas. Put the onions and peppers on a rimmed baking sheet and sprinkle with 2 Tbs. of the oil, 2 tsp. of the salt, and the pepper. Turn and rub the vegetables to coat all over with the oil and seasonings.
Grill the vegetables, covered, until they have good grill marks, about 5 minutes. Flip, cover, and continue to grill until the peppers are softened and nicely browned, about another 5 minutes. As they finish cooking, transfer the peppers to the baking sheet. Reduce the heat on the gas grill to medium low or transfer the onions to the cooler part of the fire and continue cooking until they are just tender and browned (it’s fine if they’re charred in places), about another 8 minutes. Move to a cutting board and let cool. Scrape the skins off the peppers if you like. Coarsely chop the peppers and onions and toss in a large serving bowl along with the corn, bacon, cilantro, and chili powder.
Put the potatoes in a large pot, cover with cold water by a couple of inches, stir in the remaining 2 Tbs. of salt, and bring to a boil. Reduce to a simmer, cover, and cook until the potatoes are just tender, 12 to 15 minutes. Drain and toss with the grilled vegetables, the remaining 1/2 cup plus 2 Tbs. oil, and the vinegar. Season with salt, pepper, and more vinegar to taste. Let sit for at least 30 minutes and up to 2 hours at room temperature before serving.