Fill a tall glass with ice. Add the tea syrup, gin and lemon juice. Stir with a cocktail spoon. Top off with the vanilla bean soda. Serve garnished with a lavender sprig if desired.
Combine the tea, sugar and 1 cup water in a small saucepan. Place over medium heat and cook, stirring often, until the mixture comes to a boil and the sugar is dissolved. Reduce the heat to low and simmer for 5 minutes. Turn off the heat and allow the tea to steep for 5 minutes. Strain through a fine-mesh strainer. Let cool completely before using.