Thing is, vinegar is produced by bacteria that only eat alcohol and alcohol is mainly produced by microbes that only eat sugar. So the ’strength’ (level of acetic acid) of the final vinegar will depend on the level of sugars in the stuff you start out with. I have a feeling most of the sugars will have left with the cider but there is nothing that says you can’t add sugar (or honey or molasses…) and get some kind of vinegar in the end. If it tastes good and sour it will be absolutely safe to use for human consumption.
