Combine rhubarb and sugar in a bowl, mix well and cover.
Place it in the fridge, taking it out to stir well twice a day, for three days.
After three days, strain the fruit over a clean bowl, pressing firmly to release as much syrup as possible. Discard rhubarb.
Add vinegar to the syrup, mix well, and pour into a clean, sterilised bottle.
Refrigerate for one week, before using.