Vinegar is identified as dilute acetic acid, CH3COOH, dissolved in water alongside the residue of whatever wine or ale it was made from, an identification with essentially no historical ambiguity since the substance has been continuously produced and used, under the same name, from antiquity to the present. Where the alchemical account differs from the modern one is in mechanism rather than identity: early modern writers described vinegar's formation as a further fermentation of wine, a natural continuation of the same process that had made the wine alcoholic in the first place, whereas modern microbiology recognizes it as a distinct biochemical process, the oxidation of ethanol to acetic acid carried out by acetic acid bacteria rather than by the yeast responsible for alcoholic fermentation proper.
The alchemical framing was not wrong about the observable sequence, wine left in air does turn to vinegar, but it collapsed two biologically distinct transformations, fermentation and bacterial oxidation, into a single continuous process, a simplification the underlying chemistry does not support. The distinction matters beyond pedantry: vinegar making and wine making require different, incompatible conditions, wine fermentation wants to exclude air while vinegar production needs it, a practical difference vintners and vinegar makers managed skillfully long before anyone could explain why the two processes needed opposite handling.