Combine 2 cups granulated sugar and 1 cup water in a small saucepan. Heat over medium, stirring occasionally, until sugar fully dissolves and mixture just reaches a gentle simmer (do not boil vigorously). Remove from heat after about 4–6 minutes.

Add zests and steep: Stir in the lemon and lime zests into the hot syrup. Cover the pan and let the mixture steep for 20 minutes off the heat to extract bright citrus oils from the zest.

Strain and flavor: After steeping, strain the syrup through a fine mesh sieve or cheesecloth into a clean bowl or jar to remove the zests. While the syrup is still warm, stir in 1.5 tsp citric acid, 2 tbsp fresh lemon juice, 2 tbsp fresh lime juice, 1 tsp vanilla extract, and 2 tbsp cold water (if you want slightly looser consistency). Stir until the citric acid fully dissolves.

Cool and store: Allow the syrup to cool to room temperature, then refrigerate in a sealed container. The syrup keeps for up to 2 weeks refrigerated.

To Assemble (per 1 serving)

Prepare glass: Fill a highball or Collins glass with 1 cup of ice cubes.

Measure syrup: Add 2 tablespoons of the prepared lemon-lime syrup into the glass. Taste and adjust: if you prefer a sweeter, more intense lemon-lime flavor, increase to 2.5–3 tbsp; for lighter, use 1–1.5 tbsp.

Add carbonation: Slowly pour 10 oz chilled carbonated water over the back of a spoon or down the inside of the glass to preserve bubbles. Stir gently once with a long spoon 2–3 times to integrate.

Garnish and serve: Squeeze the lime wedge over the drink if desired, drop it in as garnish, and serve immediately with a straw.