Add vinegar, sugar, water, and sriracha to a small saucepan. Heat and stir until sugar has dissolved.
Mix together cornstarch and water, and then add the cornstarch mixture to the pot. Heat until simmering, stirring occasionally. The sauce will thicken up, and clear after about five minutes.
This will keep in the fridge in a jar for up to two weeks. We often use it straight from the pot.