1. Mash berries with a wooden spoon in a large bowl. Stir in vinegar. Cover with plastic wrap, and let stand until liquid turns deep red, 24 to 36 hours.

2. Line a fine sieve with cheesecloth. Pour berry mixture through sieve into a medium saucepan, pressing to release juice; discard solids. Stir in sugar.

3. Bring berry mixture to a boil over high heat. Cook 10 minutes. Remove from heat. Skim foam from surface. Let cool completely. Cordial can be refrigerated in an airtight container up to 4 months.

4. To serve, fill each glass with ice; add about 3 tablespoons cordial, and top with about 1 cup seltzer.