Fyi, Only time I use vinegar/cider is in wasp and hornet trap bait. Never for my bees.
If dealing with an outbreak of idiopathic brood disease, one may be inclined to try various things. As with IBD, the specific cause is unknown or not lab diagnosed at the time. In such a case I could understand how a vinegar/cider sprayed onto the bees and combs may be helpful. IBD can have combined and compounding causes; bacterial, fungal, viral. Most of those pathogens can be controlled (killed)by low PH, the acid. The acid spray works on contact to reduce pathogen load. I suggest that a dose of OA or Formic will have the better acidic effect with added bonus of mite control. (NB formic is like a very strong concentrated vinegar.)
The case Jim described; we see occasionally. Lab analysis always reports back as EFB exacerbated by viral load (eg; sac brood). A shot of OAV, OA drench or Formic clears it up, along with a good flow or feed syrup. The alternate of the vinegar/cider acid spray as suggested by the inspector can be helpful if one is not setup with those other treatment methods. However, I hope the comments made steers one to reach for the OA instead.
As for adding cider/vinegar to syrup to make the feed bucket more acidic to be supposedly better for the bee, I shall have to call that out for what it is - a bucket load of BS or perhaps simply misunderstanding of the purpose and method. To clarify, use the vinegar/cider remedy as a spray-drench treatment for cases of IBD. Not intended a feed additive.