Make this slushie with a big, bold red wine, like a Cabernet Sauvignon or Zinfandel. It will have better color and a more well-rounded flavor than with a lighter red.
In a saucepan, cook berries, 1/4 cup water, sugar, and vinegar over medium-high heat until fruit has softened, about 5 minutes, smashing berries with a potato masher. Strain through a fine-mesh sieve, pressing as much liquid out as possible. Discard seeds. Cool liquid.
Combine juice and wine in a quart-size zip-top bag. Seal bag and freeze overnight. To serve, massage the bag to break up mixture. Tip slushie into a glass. Add garnish and serve immediately.
Make a bigger batch in a gallon-size zip-top bag for serving at a party.