Heat broiler with a rack 6 inches from heating element and another rack below. Season pork with salt and pepper. Wrap bacon around pork in a single layer, stretching slices so ends overlap on bottom. Transfer to a rimmed baking sheet, seam-side down. Lightly drizzle oil all over sheet around pork. Add polenta and onion to sheet in a single layer; turn to coat with oil. Season with salt and pepper. Broil 5 minutes. Flip pork; broil 5 minutes more.
Meanwhile, place asparagus on another rimmed baking sheet, drizzle with oil, and season with salt and pepper.
Reduce oven to 375 degrees. Flip pork seam-side down again. Place sheet with asparagus on lower rack. Roast until asparagus is crisp-tender and a thermometer inserted in middle of pork registers 140 degrees, about 12 minutes. Generously drizzle asparagus and onion with vinegar. Let pork rest 10 minutes before carving and serving, with polenta and vegetables.
