Place the onion into a 6 quart Instant Pot®. Cut the pork in half lengthwise and season with salt and pepper. Sprinkle the pork with the brown sugar and chili powder. Place the pork into the Instant Pot®. Stir the soup and vinegar in a small bowl. Pour the soup mixture over the pork.
Lock the lid and close the pressure release valve. Pressure cook on High pressure, setting the timer to 40 minutes (timer will begin counting down once pressure is reached- it takes about 15 minutes). When done, press Cancel and use the quick release method to release the pressure.
Remove the pork to a cutting board. Select the Saute setting. Cook the sauce mixture for 5 minutes or until reduced. While the sauce is reducing, shred the pork. Return the pork to the cooker and stir. Season to taste. Serve the pork mixture on the buns.