Generously prick each potato with a fork (about 8-10+ fork pricks should do it - this is to allow the vinegar to permeate the potato)
Place them into a medium pot, and cover with the vinegar, and a good sprinkle of salt. If the vinegar doesn't cover them, top it up with water. Turn the heat to medium high, and allow the potatoes to bubble away for a good 15-20 minutes, size dependent. Preheat the oven to 180C or 356F.
Once the potatoes are completely cooked and a knife slides through the biggest potato easily, turn the heat off. Allow them to sit in the vinegar and soak up a little extra flavour for 5 or so minutes. After this, drain them in a colander, and allow them to steam dry and cool a little, for around 10-15 minutes.
While the potatoes are cooling, coat the bottom of a large baking tray with the olive oil, and place it into the oven.
Once the 10-15 minutes are up, use your hand or a utensil to roughly (but gently) smash the potatoes. The more edges exposed, the more crispiness! Carefully add the potatoes to the hot oil, spread evenly. They should sizzle a little on impact, which will start forming a crispy base straight off the bat.
Now for some patience. For maximum crunchiness, cook the potatoes for at least an hour, if not an hour and 20 minutes. You can take them out whenever you need to, but the crunchiness is worth waiting for, if you have the time. Note that the time is highly dependent on the size of your potatoes: bigger pieces will need more cooking time.
Straight out of the oven, sprinkle the potatoes with the salt of your choice. Serve with whatever you fancy. I like mixed herbs and some rocket, but I also like a big bowl of Kewpie mayo and not much else.