Question

Would a person taking vitamins and supplements be at the same cognitive and physical level as someone who just ate well and got all the required amount of nutrients without taking supplements or vitamins ?

Answer 1

Not if the person taking supplements and vitamins weren't eating as healthy as the other guy. But if he was eating well (ie. getting his need of proteins, fats and sugars covered) then yes. He would get all the vitamins and minerals he needed through his diet. (unless of bourse there's an underlying medical reason, like a vitamin insufficiency, in any of the cases)

Answer 2

not necessarily, and probably not.
vitamins - as food supplement are less regulated than medicines or food for that matter. the FDA does not enforce companies to test for the bioavailability (how much from what we ingest actually gets to our system). we may think we are taking vitamins - but not necessarily.
secondly - there are publications that excessive doses of vitamins INCREASE the risk for certain cancers.