IMHO most people can pretty well do without vitamin supplementation for a few days without harm. A week or three without vitamins, even assuming a limited and nutritionally inadequate diet, should do little harm in most cases.

That said, according to some reports most Americans are not meeting even their minimum daily requirements of calcium, magnesium, zinc and vitamin-D.

A friend who works at a mental health admissions clinic says that it is standard practice for them to dose homeless ranters with oral B-vitamins and fish oil. He says that even a single dose gives a great improvement in mental health status in almost half the cases.

For people who are chronically short on these compounds or people with metabolic imbalances that make them need more than normal a lack of these can cause them to act erratically, becoming depressed and/or paranoid. I have noticed that if I take fish oil and a large dose of mixed Bs when I'm feeling down I feel much better in a few hours. Could be real or placebo effect. Gift horse - Mouth. YMMV.

The good news is that full array of minerals, Bs, fish oil and plain old multivitamins are cheap and compact. A couple of multivitamins can be shoehorned into just about any survival kit.

Long term vitamin-C and D may be issues even for people who have no underlying issues. People in cold climates may have problems getting enough sun exposure to keep up with their needs for vitamin-D and when things get tough the first thing to fall from diets are the fresh fruits and vegetables. At a minimum a bottle or two of Vitamin C and D should be in any long term kit.

More widely, especially for kids, a multivitamin could help keep them healthy by making up for an unbalanced diet. Couldn't hurt the adults either. Good multivitamins are available for a couple of cents each. Cheap insurance.

I keep a supply of multivitamins, multi-mineral tablets (calcium, magnesium and zinc combined), B-50, C, and fish oil in stock for long term contingencies. They may or may not make any difference but it gives you some options and something to work with to fight off known long term nutritional problems like C and D deficiencies. Not too shabby in my book for such a small investment.