Place all the ingredients into a large sauce pan and bring to a simmer over medium heat. Stir occasionally and mash the berries gently as they soften. Reduce heat as needed to keep the liquid from reaching a boil.

Continue to simmer until the liquid has reduced to a little less than half the original volume. The vinegar should thickly coat the back of a metal spoon.

Strain the reduction into a bowl through a fine mesh sieve. Stir and mash to work it through then discard or repurpose the remaining seeds.

Store the reduction in a clean jar or glass bottle with a tight fitting lid. It should keep in the refrigerator for weeks.