The problem with saying that vitamins are useless is that it's assumed that all the people who take vitamins also eat a healthy, balanced diet. Sure, if you eat right, you are very likely already getting what you need, but not everyone does. I hardly ever eat veggies or fruit these days, at least not enough to qualify toward a balanced diet. If all I eat in an entire day is a serving of spaghetti and a snack of yogurt, I am very obviously not getting all the nutrients that I need.
One of the issues I see with many vitamins is that they put far too much in them. You do not need 1000 mg of vitamin C, as the average person only needs between 65 and 90 mg per day. Perhaps if companies would stop using mega doses for their supplements, there wouldn't be so much of an issue with negative health issues down the road from overdosing on vitamins.