In a medium saucepan, mix the water with the stevia and vanilla extract.
Add the cranberries. Bring to a simmer over medium heat (this should take about 3 minutes).
Lower the heat to medium-low to keep a consistent simmer. Cook, uncovered, stirring occasionally, until the cranberries are soft and starting to burst, about 10 minutes.
Remove the saucepan from the heat. Taste the sauce and decide if its level of tartness suits you or if you’d like to add more stevia. I sometimes mix in an additional 1 teaspoon of stevia at this point (which equals ⅓ cup sugar).
Cool to room temperature before serving.
If using a granulated sweetener instead of stevia, use a powdered sweetener to prevent it from crystallizing as the sauce cools down.