Add vinegar, sugar, and 1 ounce of water in a saucepan and heat over high heat for about 15 minutes, stirring occasionally.
When the caramel begins to turn medium brown, lower the heat to medium and very carefully and gently add 2 ounces of cold water. This may cause the water to boil at first, so watch out for splashes!
When the water is incorporated, return the heat to high and let heat for one more minute, stirring continuously.
Stored in an airtight container in the fridge, this syrup should last a month or two pretty easily … but I doubt it’ll last that long!