This all sounds a bit like the sort of marketing you'd get from vitamin pill salesmen and nutritionists - do you have any links to actual academic research/papers to back this up at all? i.e. the claims that there are now less vitamins etc.. in veg and that due to this vitamin pills would be recommended.
ACtually it sounds like, basic logic. Farmers HAVE to rotate and leave parts of their land empty aswell as use lots and lots of fertilizer and added things to the soils to keep them "healthy". Why, because the turnover, massive crop numbers, larger crops, taller wheat etc, etc, if they don't rotate the fields and "rest" fields sometimes for a good few years the crops are far worse, and essentially useless, why, because the soil is quite literally so low on the things crops need to grow they don't grow effectively. That is catagorical proof that due to the intense farming we do now the soil is less nutrient rich and the crops simply aren't as good quality in the massive scale farming we do now as it used to be.
Ask any farmer in the world today and they will tell you the vegetables grow faster, bigger, but with less quality because thats what the market demands of them.
As for Freefaller saying a healthy adult shouldn't need to add anything extra, well firstly FF is a guy that eats about a gazillion calories a day, that ISN'T a normal diet, most likely you'll be getting excess vitamins and nutrients because you eat more than you need as you're trying to grow. Someone on a normal diet quite literally won't be getting the same nutrition as what they were getting 100 years ago, and no our bodys, genes and needs don't change in such a short period of time.
What is a "normal" diet these days is nothign remotely like a normal diet even 100 years, ago, which likewise is incredibly different to that of a normal diet 5k years ago. The change in our genetic make up, nutritional needs and what we require hasn't particularly changed significantly in thousands of years, yet our diets have change drastically, the manor in which we grow, culivate of breed our food has changed immeasurably.
Egg's aren't as good as they once were, chickens were few and far between and feed a more natural diet of non processed seeds which resulted in a very high omega 3 content, these days the battery farm chickens feed on processed cheap seed that itself is mass produced with little quality produce egg's with far far lower omega 3 content.
All I ever see in the sports forum is people saying if you eat a normal diet you don't need anything extra, yet there is no normal diet, just about no one on earth eats a "natural" healthy diet, infact due to the quality of the foods you can get, from most of the meats, egg's, dairy, vegetable, its all mass produced, its all done to make the most profit, none of it is as high quality as it was. But the most hilarious part is, most of the people in the sports forum don't eat a normal diet anyway, most are dieting/bulking and neither are normal sustainable diets.
As for the people who took vit's and say, I had them for 3 weeks, it made no difference. i can't say why, I can't say what you took, I can't say what you might be deficient in or what your diet is, but neither did really anyone who said they didn't work or help.
But as I said, what you eat now can effect how your body ages and deteriorates 40 years from now. Personally I'd prefer to take some vits, eat a healthy diet, cut out the crap now and stick to a clean healthy diet now, than look back in 40 years with all kinds of medical problems wishing i HAD been eating that way for 40 years while I break my hip kicking myself for not doing so.
People are short sighted, oh, I don't see a benefit right now this instant, I'll do something about it when its a problem. In lots of medical cases its simply too late by then. You can prevent diabetes pretty damn easily by eating right now. HOwever you can eat badly and not feel remotely bad till you get diabetes, but at that point its simply too late. Not doing anythign about your diet because you don't feel instantly better on a so called healthy diet, is about the most stupid thing you'll ever end up doing in your life.