In a medium saucepan, combine the apple cider vinegar, white vinegar, brown sugar, ketchup, crushed red pepper flakes, black pepper, salt, and hot sauce.
Place the saucepan over medium heat and whisk continuously until the brown sugar and salt have completely dissolved into the liquids.
Bring the mixture to a very gentle simmer. Do not let it reach a rolling boil, as you do not want to evaporate or reduce the vinegar.
Let the sauce simmer gently for about 10 minutes. This crucial step allows the heat from the red pepper flakes to fully bloom and infuse into the vinegar base.
Remove the pan from the heat and let the liquid cool entirely. Transfer the thin sauc