Add the rice vinegar, sugar, water, and sambal oelek to a small sauce pot. Heat and stir the mixture over medium heat until the sugar fully dissolves.

Stir the cornstarch into 1 Tbsp water until dissolved, then pour it into the sauce pot with the sweet chili sauce. Continue stirring and heating until the mixture comes up to a simmer, at which time the cornstarch will thicken the sauce and go from appearing cloudy to clear.

Use the sauce immediately, or refrigerate and use within one week