Position racks in the top and bottom thirds of the oven and preheat to 375 degrees . In a bowl, combine the dates and breadcrumbs, working the mixture through your fingers to separate. Mix in the beef. Mix in the chopped onion, bacon, egg, worcestershire and vinegar; season with salt and pepper. Divide into 4 mounds and, on a baking sheet, shape into loaves. Place on the upper rack and bake for 45 minutes.
Meanwhile, on another baking sheet, toss the sliced onions with the butter and season with salt; spread out evenly. Bake on the lower rack, stirring occasionally, until golden, about 30 minutes. Serve the meatloaves with the onions.