In a large bowl, mix together the raspberries, sugar, and lemon balm. Mash the berries with a potato masher.
Put the bowl in the refrigerator and let stand at least 12 hours or up to 2 days. Give it a stir now and then.
Put a double layer of cheesecloth over a fine mesh sieve (or use a jelly bag). Strain the berries through the cheesecloth. After the most of the liquid has seeped through, pick up the edges of the cloth and squeeze to get as much of the liquid from the berry mixture as is possible.
The liquid should measure about 1 Cup. Add an equal amount of vinegar to the berry liquid and mix well.
Pour the finished shrub into a clean glass bottle with a tight fitting lid. Refrigerate for at least an hour for the flavors to meld and mellow before serving.
