I've seen the ads, too, and can't imagine how it could possibly have any effect except skin irritation. Cleaning strength (30% strength as opposed to 5% for kitchen variety) vinegar will start to burn after a minute on your skin. But size increase?
You got me curious, so I had to go look for the ads. But I noticed that the products themselves contain no apple cider vinegar whatsoever. The phrase "Apple Cider Vinegar" is just randomly splattered all over the text, as a heading to each paragraph, etc.
I suspect the reason is to make search engines find the penis enlargement sites, while people are looking for something else. People often do search the phrase "
Apple Cider Vinegar," as for generations supposedly miraculous properties have been attributed to this substance by magazines sold at supermarket checkstands and read by bored women. Which is why half a century ago, my pea-brained grandmother force-fed it to me as a child. (Then again, I do have a big dick...)
By the way, apparently soaking your dick in vinegar can cure male yeast infections, and possibly jock itch. But it will not make it larger. Oh, and possibly Apple Cider Vinegar can make rats with liver disease produce better sperm, too:
Beneficial effect of apple vinegar on reproductive parameters in male rat model of nonalcoholic fatty liver disease - PubMed