In a medium saucepan, combine the granulated sugar, light brown sugar, and water. Split the vanilla beans and scrap the seeds into the pan. Whisk to combine and add the pods (if using vanilla extract, do not add until after simmering the syrup).
Bring the mixture to a boil, then reduce heat to a simmer and cook, whisking occasionally, until the sugar is dissolved and the liquid is starting to thicken slightly, 10-15 minutes.
Remove from heat (if using vanilla extract, stir in at this point) and allow to cool to room temperature. Discard the pods, transfer to an airtight container and refrigerate for up to 2 weeks.