Mix the vanilla extract, coffee and sweetener with the water in a small saucepan. Bring to the boil and simmer until the sweetener dissolves before removing from the heat.
Allow the water mixture to cool down before adding the vodka. Pour into a glass jar and store in a dark cool place.
Please take note of the type of sweetener specified in the recipe as a stevia erythritol blend is a lot sweeter than just using erythritol or xylitol. Note also that you can vary the amount of sweetener used but please taste the finished mixture before adding any additional sweetener.