-You can use purchased vanilla syrup.
-Make your own vanilla syrup by bringing a 1/2 cup of sugar and a 1/2 cup of water to a simmer, stirring to dissolve sugar. Remove from heat and add 2 tsp vanilla extract or fresh vanilla beans from 1/2 a vanilla pod. Allow to cool and then strain if using fresh vanilla beans.
-If no vanilla syrup is possible, sweeten to taste with sugar and add a splash of vanilla extract.