Stir together the vinegar, soy sauce, chilli paste, garlic, ginger, red chilli flakes, black pepper, sugar, spring onions and onion in a large bowl. Mix in the pork slices, mixing well until completely coated. Place into a resealable plastic bag, squeeze out any excess air, seal and marinate in the fridge at least 3 hours.
Heat the oil in a large frying pan over medium-high heat. Add the pork slices in batches, and cook until no longer pink in the centre, and lightly browned on the outside, about 5 minutes per batch. Be careful when cooking the meat, the spicy fumes will hurt your nose!