Wikipedia says one cause of dandruff is yeast-like fungus. Perhaps PH plays a role in that. Also vinegar has cleansing properties and may be less drying than detergent, and dry skin is another cause of dandruff.
https://www.urmc.rochester.edu/encyclopedia/content.aspx?con...
Maybe that's because vinegar isn't a staple in the hair care aisle at Walgreen's.