You do not understand. The Vitamin itself is fat soluble. This has nothing to do with how it is packaged or supplied. You can't make the Vitamin be water soluble like the B Vitamins or Vitamin C.
Being Fat soluble simply means that it can be stored in the body in the liver as opposed to being supplied or synthesized on a daily basis as water soluble Vitamins must be. The problem with it is that the Vitamin A stored in the fall from green grass and freshly cut hay runs out long before the new season Vitamin A is available. That is why horses become deficient this time of year.
Good quality 'fortified' feeds supply it. Those horses do not get Rain Rot. When I was fortunate enough to be able to get custom made feed pellets, I always ordered it with 10,000 IU of Vitamin A per pound in it. Then I did not have to worry about where it was going to come from.
Mare Plus, which is the supplement we recommend, supplies 100,000 units per day when fed as directed. This is about the same amount supplied by the oral Vitamin A injectable if it is used once per week. We never use that amount more than 1 or 2 weeks until our feed can supply what is needed.
As you can see, it does not exceed the amount supplied by Farnam in their product. It just corrects the problem faster and cheaper. If you don't like the idea of using the injectable form orally, just wait a little longer for the results and use the Farnam horse product in a horse's feed.
It really is not nearly as complicated as people want to make it. Many Veterinarians should be embarrassed that they do not know any more than they do about nutrition and the part it plays in good health -- but then, they would make a lot less money than treating all of the problems it creates.