The NYTimes.com Well Blog suggests clinical studies are unable to find benefits among vitamin supplement-takers:

Despite a lack of evidence that vitamins actually work, consumers appear largely unwilling to give them up. Many readers of the Well blog say the problem is not the vitamin but poorly designed studies that use the wrong type of vitamin, setting the vitamin up to fail. Industry groups such as the Council for Responsible Nutrition also say the research isn’t well designed to detect benefits in healthy vitamin users.

I like the “despite the lack of evidence” line, as if the evidence is conclusive or at least compelling when shown to the consumer.