Make the ginger syrup: Combine the water and sugar in a small saucepan over medium heat. Stir until the sugar dissolves completely, about 2 minutes. Add the sliced ginger, reduce to low, and simmer for 8–10 minutes until fragrant. You'll see the liquid turn a pale gold color — that's your cue it's ready.
Strain and cool: Remove from heat and stir in the vanilla extract. Let the syrup steep at room temperature for about 30 minutes so the ginger flavor fully infuses, then strain through a fine mesh sieve into a jar or bottle. Discard (or save) the cooked ginger slices — they're pretty if you want to use them as garnish later.
Build your drink: Fill a glass with ice cubes. Add 1 tablespoon of fresh lemon or lime juice, followed by 1/3 cup of the prepared ginger syrup. Top with 1 1/2 cups sparkling water and stir gently once to combine. Garnish with a thin slice of raw ginger on the rim if you're feeling fancy.
Adjust to taste: Taste before adding more syrup — some people prefer it sweeter, others want more citrus bite. Add an extra splash of lemon juice if you'd like it brighter, or a touch more ginger syrup for depth. Serve immediately while still fizzy.
