Combine the water, sugar, orange zest, grapefruit zest, cinnamon stick, cardamom seed, coriander seeds, star anise, and vanilla bean in a saucepan and place over medium-high heat. Whisk until the sugar has dissolved and then bring to a boil.
Reduce the heat and simmer for 15 minutes. Remove from the heat and strain into an airtight container. Add the orange and grapefruit juice (should be about ⅔ cup of juice total). Mix in the browning sauce and vanilla extract. Once cooled, cover and store in refrigerator for at least 12 hours and up to 2 weeks.
To make a serving, combine the syrup, citric acid, and ice in a cup. Pour in the club soda and stir.