Add the peel and the fruit to the jar. In a pot over medium heat, combine 2½ cups of water with the sugar and heat until sugar is dissolved.
Pour syrup into the jar and add vinegar, if using. Cover the top of the jar with cheesecloth or a clean dish towel (this will keep bugs out while allowing air in), and set it in a warm place. Shake the jar every 2 days.
After a week and a half, remove the peel and fruit, and continue to let the liquid ferment. There may be white gunk along the bottom of the jar or bubbles on the surface, but don’t be alarmed; both are fine. The vinegar can be used as soon as 2 weeks into the process, but it gets better the longer it ferments.
How to use pineapple vinegar:
For a quick vinaigrette, mix 3 tablespoons of oil with 1 tablespoon of pineapple vinegar. For a marinade (especially good with pork, chicken or fish), mix ½ cup oil, ½ cup pineapple vinegar, 1 clove crushed garlic, 1 tablespoon chili flakes, a small handful of chopped cilantro, salt and pepper.