When I feed my bees I use ACV at the rate of 1 Tbsp per gallon. I've been doing it for over 50 years. ACV, beside simple syrup, is the only treatment of any kind I use on my bees, my hive losses have been low, hive vitality has been high, as has been production per hive.
Since both ACV and natural oils are deemed to be in the "Crackpot" scheme of beekeeping scientists have not given either any serious considerations. Most of the scientific studies I've read, that expound great discoveries, have been of such a short duration as to be invalid in it's results. Usually further extended studies have proven the conclusions of the short studies in error. A study of 4-8 weeks really proves nothing, a study over 2-3 years would have much more validity but is often not done due to lack of "funding."
As a result may scientific fallacies continue to be prepetuated and "crackpot" practices that have proven helpful through years of use continue to be looked at as quackery, or as Finski says, "magic.".