Combine the fish sauce, rice vinegar, sugar, and red chili flakes together in a medium sized pot over medium-high heat. Whisk to dissolve the sugar, and bring the mixture to a boil (about 10 minutes). Keep an eye on the pot – if the heat is too high there is a chance it may boil over if you aren’t careful (not that I know from experience or anything…). In the meantime, whisk together the cornstarch and the water in a separate bowl.