Can skin so soft bath oil and vinegar keeps flies off horses?

Never use human hair or Skin Care products on a horse without a equine vets approval as they may cause problems and are not PH balanced for a horses skin. Skin So Soft tend to attract gnats and flies rather than repel them, so I would not advise using it. Vinegar can help to 'shine up' the coat and prevent some pests from bothering the horse. Your best bet however would be a horse specific insecticide shampoo or fly spray.