Staying healthy in a survival situation should be at the top of your preparation to-do list. By maintaining health in an extreme circumstance you can better insure your own survival and that of your families.
Along with attention to proper hygiene, the key to staying healthy when you are relying on your own resources is eating proper nutrition. The absolute best way to get good nutrition is by providing sources of fresh fruits and vegetables to you and your dependents in every situation. An MRE is not going to get the job done and neither is a conventional vitamin.