Set the vanilla extract aside for later. Add the allulose, water, and salt to a small saucepan and stir to combine.
Place the saucepan over high heat and bring the mixture to a boil.
Once boiling, reduce the heat to medium-low and let the syrup simmer for 10 minutes.
Remove the saucepan from the heat and stir in the vanilla extract.
Use the syrup right away, or allow it to cool completely before transferring to a syrup bottle or airtight container.
Store in the refrigerator for up to 3 weeks.