I feed Cider Vinegar to my horse who has arthritis and it really seems to help him: however if anything I would say the Cider Vinegar deterers him from eating....he wont take it in the summer when he's on half a scoop of chaff so I have to leave it until the winter. On winter rations he doesn't seem to mind it so much....in my experience I would only use it as a joint and coat supplement, I cant imagine it would encourage a fussy feeder.
I use it for all the above reasons and I believe it helps in the battle against flies. I buy it from the wholesale, in bulk.And the horse who can be fussy, loves it.
I don't think it is meant to tempt a fussy feeder but rather stimulate their appetite so they are more hungry and therefore eat more. If the horse doesn't want to eat much then putting it in the feed won't make much difference ('cos he won't have eaten it!)