Vinegar is a good fly repellent but it doesn't last a long time....seems to evaporate into thin air, so you might find you need to re-do thru-out the day.
Of course that said, the commercial sprays don't last a whole day either.
It's nice to use on the babies though it's free of chemicals and nice for people too, as I am allergic to commercial fly sprays.
If your horses will tolerate it ... I make a 'salad dressing' of corn oil and apple cider vinegar and drool it over their hay. Depending on
the time of the year the ratio changes to more vinegar or more oil.
More vinegar in the fly season / more oil in the winter and spring to help encourage them to drop those heavy winter coats and it seems to help with the dry skin itchies
Once the blood stream has a steady supply of vinegar in it you'll notice the horses are less affected by flies and mosquitoes as their
blood becomes less palatable to the insects.
I've never had a horse refuse it but others have told me their horses didn't like it.
It needs to be introduced slowly, of course.
I also introduce a small drip of apple cider vinegar, to water buckets, in advance of knowing I am trailering a horse off the farm.
They tend to not turn up their nose at 'foreign water' if you then give them a bucket at the show or where ever, and put a drop of apple cider vinegar in it.