Combine one punnet of blackberries, sugar, vinegar, and ½ cup water in a small saucepan and simmer for 3 minutes, until the berries are softened and the mixture is syrupy. Stir in the vodka and Grand Marnier, then set aside to cool.
Pour into tall glasses and top with chipped ice. Add the tonic water gently, then garnish with the remaining berries.