There's basically no evidence anywhere that vitamin supplementation does anything at all for generally healthy people who don't currently suffer a specific deficiency.

If you do find some studies that find some evidence for vitamin supplements doing anything (for generally healthy people), please let me know!

See https://www.youtube.com/watch?v=NmJsCaQTXiE for a quick intro. (Sorry, don't have enough time to find exactly the right video and/or paper.)

The person you're answering to didn't say anything about "healthy people who don't currently suffer a specific deficiency". I don't think multivitamins are meant for this population anyway, it's for people who know they're prone to deficiencies or even know they have one specific issue (e.g women and iron, dark skin and vitamin D)

I've anecdotally heard of iron supplementation working for some women.

However, if I remember right, even for the seemingly clear case of dark skinned people vitamin D supplementation basically has no evidence. But I'd be happy to see any randomised controlled study (or similar) that says otherwise!

There's plenty of observational studies that people with lower than average vitamin D also have various other health problems. But I don't know of any study that shows that vitamin D deficiency helps eg dark skinned people (or any group at all).