I have read studies from every side of public opinion on vitamin use. There is no conclusive evidence that proves anything because there are too many factors for a true controlled study to be done.
Things like genetics, diet, excercise and stress levels are not controlled in these studies. And if you think about it, science will never find enough similar humans to perform a controlled study.
This we do know...food lacks the nutrition it once did. Most fruits and vegetables are grown in nutrient depleted soil and are the result of genetically engineered species. This is true whether we're discussing plants or animal foods.
Secondly and most importantly, taking vitamins and supplements is part of a healthy lifestyle. Users have nothing to lose by taking them and the positive attitudes gain through such a lifestyle could be just as beneficial as if vitamins cured disease.
If you know someone who leads a healthy lifestyle of good eating, excercise and vitamins, you probably notice that the quality of their life is typically better than those who eat crappy are lazy and don't take vitamins.
These stories don't don't affect vitamin sales. The LA Times recently published a similar article and it had no effect upon sales.