Boil the potatoes in water with vinegar for 10 minutes.
Drain the water from the potatoes and place them on a rack, where they can cool/steam off for 5 - 10 minutes. This removes some of the moisture, which is your enemy in the quest to get the potatoes crispy.
Give the cooled potatoes a press, for example with a potato masher. You don't need to press too hard, just enough to break them a little at the top (and bottom if they're on a rack). The idea is to create more surface area that can become crispy.
Season the potatoes with salt and pepper.
Pour oil into a small bowl and toss the potatoes in it. If you're patient, toss each potato individually to ensure they all get well-coated in oil.
Place the potatoes in an ovenproof dish and put the dish in the oven.
Cook them at 180 degrees Celsius (356 degrees Fahrenheit) (fan-assisted) for 30 - 45 minutes. Every 15 minutes, open the oven, give the potatoes another little press, and baste each potato with a bit of oil from the dish. Continue this until they are golden and crispy. It took me about 45 minutes, but you can stop after 30 minutes - I just went all-in on crispiness :-)
Take the potatoes out of the oven when they are done. Place them on paper towels to absorb excess fat and serve immediately.