Combine vinegar, sugar, garlic, ginger, sriracha and chili flakes in a small pot and bring to a simmer over medium heat. Cook for 5-7 minutes, or until mixture begins to turn syrupy, then reduce heat to low. Mix cornstarch in 1/4 cup of water and stir into sauce. Bring back up to a simmer — sauce should thicken substantially. Remove from heat and serve. Can be kept tightly covered and refrigerated for up to a week.