If the sauce is too thick, next time you make it combine the first four ingredients and bring to a simmer over low heat, whisking constantly. Once the mixture starts to simmer, remove from heat and add vanilla. The syrup should be less thick than if you make a simple syrup (boiling the water and sugar until sugar is completely dissolved) first. If the syrup isn't thick enough, you can try dissolving a tablespoon of cornstarch in a little water and adding it to the sauce. Let it simmer lightly until it reaches the desired consistency.