The fact that vitamins A, E, and K are fat-soluble is key to why an oil is a practical delivery method for them. Fat-soluble compounds dissolve in and are carried by oils or fats, not water. This means when these vitamins are present in a plant oil like sea buckthorn, they are already in a form that is compatible with the skin's natural lipid barrier and can be effectively delivered.
For practical use, this oil-based delivery allows these compounds to remain stable and active within the product. When applied, they are incorporated into the light lipid film the oil creates on the skin. This contrasts with water-based products, which might not carry fat-soluble vitamins as effectively. The combination of the oil's emollient action and the presence of these antioxidant components means you are applying both the carrier (the nourishing oil) and the active compounds in a single, matched format that is traditional for herbal oils.