Preheat oven to 450 degrees, with racks in upper and lower thirds. Divide bacon between two rimmed baking sheets; cook until browned, 8 minutes. Drain bacon on paper towels, reserving sheets. Crumble bacon into bite-size pieces. Add potatoes to sheets, season with pepper, and toss to coat with bacon drippings. Cook until potatoes are brown on bottoms, 15 minutes, rotating sheets halfway through. Remove sheets from oven and flip potatoes with a metal spatula.
In a bowl, combine beans, capers, and oil; divide between sheets with potatoes. Cook until potatoes and beans are tender, about 15 minutes, rotating sheets halfway through. Transfer potato mixture to a large bowl; toss with bacon, scallions, and vinegar.