The answer: Both. It all depends on the quality of vitamins that you are using.
Throughout my undergraduate career, I was told by some professors that vitamins give you expensive urine. The reason they said this was because most vitamins are created from inexpensive materials. The poor quality vitamins are not absorbed properly, so the majority of the vitamins gets eliminated as waste (urine). Some joke that vitamins do nothing but give you "expensive urine".
At the same time I was learning this, I had a friend mention to me that he takes a multivitamin called Coreplex. That poor fella... I basically told him vitamins don't work, and I completely brushed him off. I also argued that I could obtain all of my nutrients and vitamins from my daily diet, and I did not need to supplement.
Being the science nerd that I was, I decided to investigate. One of the first things I found about the Coreplex was this: About Coreplex. It explains that yes, most vitamins are not absorbed properly, but when you take a product that is HIGH QUALITY, it will be absorbed. At this point, I was almost convinced, but then I wondered 'What makes these products high quality?' I then found the SciMed board: SciMed Board. This is a group of doctors and experts that ensure the products are safe and effective. Lastly, I watched this video, and realized I could no longer obtain all the necessary vitamins and minerals from my diet: http://youtu.be/--EwDG2UBPo
After performing my research, I decided I would give the product a try. I started taking the MNS MAX 3. Its essentially a dump truck of nutrition. Its little packs that you take at certain times of the day. After a few days on MNS MAX 3, I realized I FELT different. I had more energy throughout the day, and I just felt HEALTHY!
I cannot describe why the products work. I also cannot explain on a molecular level why we need them. All I can tell you is that I feel so great since using these, that I will never, ever go without them again.
Before I go, please note: as a future healthcare provider, I need to emphasize the importance of consulting with your Health Care Provider before adding supplements into your daily regimen. Your Provider knows your past medical history, and their approval for the use of supplementation is extremely important to your health.