Don't forget that the fungi are ubiquitous so although you can knock them out, you need to make the environment (i.e. hoof) as adverse for them as you can or they will quickly reinfect and you'll be back to square 1. I think the cider vinegar (which is an acid) works by creating an acidic environment. Horse pee contains ammonia so it's alkaline, the bugs that like growing in horse pee won't like vinegar so although it doesn't necessarily kill them, it will impair their growth so they don't get going enough to cause a problem because the horse's natural defences can keep them at bay. Also getting the feet trimmed to get rid of as much dead tissue & as many crevices as possible will help a lot. Sometimes these issues can be linked to issues elsewhere, e.g. diet/gut function, so feeding a probiotic like Pink Powder might help.