Taking vitamin pills are a great idea if you suffer a genuine vitamin deficiency. Whether that's due to some chronic illness or special diet (vegans, I'm looking at you), there are those who need them and absolutely should be taking them.
The overwhelming majority have no need of them, however. If you are already reasonably healthy (and that's a surprisingly low barrier), vitamins are going to do nothing for you. Mankind evolved to deal with scarce food resources, and the body is awfully good at scavenging necessary vitamins and minerals from our everyday diets. "More is better" is never the way biology works - too little is bad, but enough is fine, and more won't help. Antioxidant vitamins do nothing.
At some point, too much will be bad for you, no matter how benign the substance appears. Everything becomes toxic at a high enough dose. Even vitamin overdoses occur, though they are very rarely fatal.
More dangerous and insidious (and the real point of this post) is the possibility that by assuming you are doing something healthy, you will then justify other unhealthy activity- smoking, exercising less, eating less well, etc. This is "risk compensation". Overall, you take vitamins (which don't help) and then go on to do something bad for you - a net loss for your health. This is bad in itself - and serves as an example of how things that defenders pass off as "harmless even if they don't work" can be of real harm.