Whisk together cornstarch and water to form a slurry and set aside.
Toast red pepper flakes in a medium saucepan over medium heat for about 3 minutes, to enhance flavors.
Add vinegar, water, sugar, minced garlic, and soy sauce to the saucepan and bring to a boil. Cook for 4 minutes, stirring occasionally to prevent boiling over.
Reduce heat to low. Slowly add cornstarch slurry to sauce, whisking until fully incorporated.
Remove pan from heat. Sauce will thicken as it cools. Allow to cool to room temperature before storing.
Notes
Nutrition information provided as an estimate only; your actual values will vary based on exact ingredients used and serving size.