It is a common misconception that a person can get the flu from a flu shot. The vaccine contains inactivated virus particles that does not cause the flu. However, the spray version does contain a live-attenuated virus, which MAY cause flu, but is HIGHLY UNLIKELY.
In addition, the flu vaccine was created so that you are protected from the virus. That is, your body will create antibodies to make you IMMUNE. It works in much the same way as the varicella vaccine for chickenpox. Thus, as long as you have antibodies circulating in your blood, you should be protected.
The side effects of the flu vaccine include some mild symptoms such as a slight fever that is less than 100F, as well as a slight reaction at the sight of the injection such as redness and mild swelling. However, you will not get the flu.
It is true, though, that the the vaccine may not cover the actual strains of the influenza virus that is spreading. The CDC and other agencies/corporations run simulations and tests through bioinformatics to come up with the most likely strains of virus to evolve. However, sometimes these studies may miss a few strains. So it is possible that with the flu shot, you could acquire the flu from a different strain.
As for vitamins, there are a lot of research that still needs to be done. In general, it is possible to get vitamin overdose, especially those that are fat soluble (vit. A, D, E, K). There is a recent study about vit E toxicity, but I have not had the time to read it. In general though, the FDA recommendations for vitamins are much less than what the body needs. There have been studies showing that a person may need the quantity of 2-4x what comes in a regular multivitamin (ie, Centrum).
That said, what vitamins may be beneficial to hikers? well, the B complex is very good for the blood supply and creating energy, vit. C and E are great antioxidants to help the immune system. Elderberry, as mentioned in a previous post, contains some very good immune properties that help against viruses, like the flu, however, i'm not quite sure the mechanism. But if you do take it, try to avoid HUGE amounts - it is somewhat related to ricin. But I have never heard of an overdose yet.
So, eat well, exercise, and maybe the greatest factor is to not smoke. Hope the info is of help