Yeah but you gotta remember "vitamins" is just a dumbed down term to refer to fats and compounds. It's not actually like food or anything nourishing for the hair. Like a lot of haircare stuff has vitamin e in it, which is supposed to help protect hair from hot blow drying damage and also make it shiny. A lot of the stuff is also moisturizers for your scalp.
I always thought the point was for the vitamins to be absorbed by your skin. Human skin absorbs all kinds of stuff so as long as the vitamins make contact it sort of makes sense?
Though I suppose for most people it won't have much of an impact unless you have a severe vitamin deficiency.