I usually use white rice vinegar.
And if you're interested, I follow this recipe strictly:
1. The pork:
I'll choose the pork belly carefully, with about 75% lean meat, about 25% fat and skin and the entire piece of belly should be about 2 inches thick on all sides. Partially boiled in boiling water, skin facing downwards for some 10 minutes. Transferred into ice cold water immediately and soak for 5 minutes. Once cooled, those bristles should come of the skin rather easily with a pair of tweezers. I'll prick the entire skin thoroughly as this step ensures the crispy skin later on.
2. The marinate:
For a kilogram of meat, I'll use 2 tablespoons of 5-spice powder, 2 tablespoons of ShaoXing wine, 2 teaspoons of salt, 2 teaspoons of white pepper, 2 teaspoons of sesame seed oil, 2 teaspoons of garlic oil. The mixture is generously coated on the meat and fats, carefully avoiding the skin. The marinated slab of meat sits in the fridge overnight, exposing the skin to dehydrate the skin.
3. The cooking:
I preheat my oven to about 250 degrees C, takes the marinated meat out of the fridge, coat a generous layer of white rice vinegar on the skin, pop the meat into the preheated oven to let it take care of any condensation. Roasting on the lower rack for at least 15 minutes, the skin will start to bubble at this time, turn golden brown but not charred. The pricking of the skin should allow enough fats to come through, I'll let the lard oil fry the skin, let it cook for another 20 minutes until you can see the entire skin surface bursting with tiny bubbles.
If the skin isn't oily, I do add a layer of lard oil prepared previously to the skin, and continue to let the meat cook for at least 30 minutes till you see those tiny bubbles.
The tiny bubbles is a signal for me to bring the meat to the top rack for final treatment, still at 250 degrees C and it's for about 5-10 minutes. Some parts may be charred but it's ok. You'll see the oil splattering out from the skin, and I only take the meat out when about 10% of the skin is charred.
4. Serving:
Scrapping off the charred portions with a serrated bread knife after the meat is cooled, chop it up and you're ready to serve.
May I ask what vinegar you use and how long do you marinate it for.