First make the shrub. Put the pineapple, sugar and vinegar into a pot and bring to a simmer. Cover and simmer on low for 10 mins. Turn off the heat, mash the pineapple with a potato masher and then let the mixture cool. Add the hibiscus flowers. When the mixture is cool, strain it well making sure to squeeze out the pineapple. Store in an airtight jar in the fridge until you are ready to use it. The shrub can be used right away, but the flavors will meld, mellow, and get better over time.