In a small saucepan, combine blackcurrant juice, raspberry juice, grape juice, sugar, and 1/2 cup water. Stir gently to dissolve the sugar before heating.

Heat to dissolve

Place the saucepan over medium heat. Warm the mixture, stirring frequently, until the sugar fully dissolves and the mixture is just below a simmer (do not boil). This should take 3–5 minutes.

Add flavorings and cool

Remove the pan from the heat. Stir in the lemon juice and vanilla extract. Taste and adjust sweetness or acidity: add up to 1 tablespoon extra lemon juice for a brighter tang or up to 2 tablespoons extra sugar if you prefer sweeter. Let the syrup cool to room temperature (about 20–30 minutes).

Strain (optional)

If using juices with pulp or any solids, strain the cooled syrup through a fine-mesh sieve into a clean jar or bottle to obtain a smooth cordial.

Refrigerate the syrup until ready to use. The syrup keeps for up to 2 weeks refrigerated in a sealed container.

Assemble Drink

Measure syrup into glasses

For one serving (about 12 fl oz / 350 ml), add 2–3 tablespoons (30–45 ml) of the prepared fruit syrup into a tall glass. For a sweeter, fruitier drink use the higher amount; for a milder drink use the lower amount.

Add ice and sparkling water

Add about 1/2 cup (4–6 ice cubes) of ice to the glass. Pour 1 cup (240 ml) of chilled sparkling water or club soda over the ice and syrup. Stir gently to combine. Taste and, if desired, add another tablespoon of syrup.

Serve

Garnish with a few fresh berries or a lemon slice and a mint sprig. Serve immediately chilled.

To make a pitcher: combine 3/4 cup syrup with 4 cups chilled sparkling water in a pitcher, stir gently, add ice and garnish as desired. Keep extra syrup refrigerated for future batches.