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I am very, very glad to hear that you are at very least a responsible vitamin taker. I know that a lot of multivitamins think it's a selling point to offer 3000% of some vitamin du jour and that worries the shit out of me.
The fact of the matter, though, is that vitamin deficiency is pretty easy to diagnose.
http://en.wikipedia.org/wiki/Vitamin
There's a table there to show you all the diseases associated with it, and a lot of those diseases are associated with starvation, not with the diets we currently enjoy. Now it is very true that the American diet is high in animal fat and deficient in vegetables and fruits, but we do not have an epidemic of beriberi, rickets, or scurvy. I think Americans get enough vitamins as it is in their typical diet.
It is true as you pointed out that our bodies are doing very poorly with a lot of the stuff we throw at them. It turns out that our body doesn't register high-fructose corn syrup as calories which in turn doesn't trigger impulses of being full. Without those impulses, we can take in thousands of calories per day without any stopping. We partially saturated harmless vegetable oils and made them trans fats, which our body possesses no natural enzymes to break down.
I think there's the false notion that because it's natural that it's good. This is not necessarily the case. But the more processed the food is, the more untested compounds we put in our bodies, the more likely it is that we are damaging our health long-term. I prefer to eat mostly unprocessed foods, and the fewer ingredients I don't recognize, the better usually. You can compare the Mediterranean diet where the emphasis is on a lot of plain unprocessed foods versus the American diet which places an emphasis on process.
And now it's my turn to ramble. At least as a vegan, you won't have a heart attack! Just be sure you're getting enough fat to keep your brain running.