Sounds like it's an experiment worth conducting (and drinking!). Some of the acid from the vinegar will go into splitting the sucrose into "invert sugar," which is a combination of glucose and fructose, both of which are fully fermentable. However, the caramelizing from the cooking process will turn some of the sugar into non-fermentables but it's hard to predict how much exactly. I'd take a wild guess and treat it like dark brown sugar (which is sucrose with a little molasses) or dark Belgian candi in your calculations. Are sugar and vinegar the only ingredients? If there's butter in there, it may impact head retention.