It depends what you mean by “work as described”. If you mean that taking a B12 supplement will increase your B12 levels and accommodate for a deficiency, then yes, vitamins do work as intended.

If you mean the claims you often see such as “vitamin C cures colds” or “B12 increases energy and focus” then you’re correct in saying that they do not work that way.

Vitamins have been shown to increase factors like energy or overall health (more than placebo) if and only if the person has a prior deficiency.

Non-vitamin supplements are somewhat different as some of them are pharmacologically active and can do things like constrict or dilate your veins. There isn’t a hard line between supplements and drugs, meaning some supplements can have a psychoactive effect as well.