Put raspberries in a bowl, and toss to combine with sugar. Cover and refrigerate overnight.
The next day, smash the raspberries into a pulp with the sugar. Add vinegar, and stir to combine. Add 6 ounces warm water, and stir to dissolve the sugar. Pass mixture through a fine-mesh sieve to remove pulp and most seeds. Store in the fridge. This mixture should last at least a month.
Recipe by Louisiana Cookin' at https://louisianacookin.com/raspberry-negroni/