It's very important to refuse to get lost in semantics. The body is a complex system. Like zoo animals we don't live in a way that's comparable to the environment in which we evolved. Our genes for Vitamin C production might have been fall victim to gene drift.
Just like the perfectly rational participants don't exist in economics the human body isn't running perfect. Both are ideals that hide the inherent ambiguity of the real world.
It's not a good idea to pretend that things that one doesn't understand can't exist.
Take Vitamin E in elderly subjects. Meydani et al published a peer reviewed paper that showed a placebo controlled double blind study various boosts the immune system in various ways:
Subjects consuming 200 mg/d of vitamin E had a 65% increase in [delayed-type hypersensitivity skin response] and a 6-fold increase in antibody titer to hepatitis B compared with placebo (17% and 3-fold, respectively)
Take Vitamin D. In a recent meta study Grant et al came to the conclusion:
One can estimate the effect of having all Europeans reach the 40-ng/mL 25(OH)D level as the product of the economic burden of each disease and the fraction of the burden that daily intake of 2000–3000 IU of vitamin D3 could reduce for some diseases, leading to reduced mortality rates and longer healthy life expectancy.
[...]That would increase life expectancy by about 2–3 years.
There no scientific reason to demonise all Vitamin supplements as pseudoscience.