Place the water, Sprite, sugars, and soy sauce in a heavy saucepan and bring to a boil over high heat. Boil until the sugars are dissolved, 3 minutes, whisking well. Remove the pan from the heat and let cool slightly, then whisk in the sesame oil, gochujang, and rice vinegar, if using. Transfer the sauce to clean jars. Covered and kept refrigerated, the sauce will keep for at least 1 month.