In a food processor, pulse 1 hamburger bun into crumbs. mix 1/2 cup of the crumbs (reserve remaining crumbs for another use) with the chickpeas, cilantro, egg, 1/3 cup chopped onion and 1/4 tsp. each salt and pepper until combined but still chunky, 3 or 4 pulses. Form into four 4-inch patties.
Cut the tomatoes into large chunks. In a medium bowl, toss the tomatoes with the remaining chopped onion, 1 tbsp. EVOO and the vinegar; season to taste.
In a large nonstick skillet, heat the remaining 2 tbsp. EVOO over medium. Cook the patties until deep golden and crisp, 3 to 4 minutes per side. Meanwhile, split and toast the remaining buns. Slather the bun bottoms with the yogurt; top each with a patty, lettuce and a bun top. Serve with the tomato salad.