Add the water and brown sugar to a small saucepan. Place over medium heat and bring to a boil, stirring occasionally. Once boiling, reduce the heat to a simmer, add the cinnamon sticks, allspice berries, and fresh ginger. Simmer for 20-30 minutes.
The mixture should thicken slight and have the consistency of syrup.
Pour the syrup through a fine mesh strainer into a glass jar or storage container. Discard the ginger, cinnamon sticks, and berries. Add the vanilla to the syrup and stir to combine. Cool to room temperature. Store in the fridge, covered, for 1 month.
This will make about 2 cups of syrup, but will make less the longer you boil it because more water will evaporate.
Notes
This recipe doesn't use molasses, but if you want, you can add a tablespoon or two while you're cooking it. This will give it a deeper molasses flavor. Store in an airtight container in the fridge.