Vitamins are necessary to the body.  You have to have them, or you suffer serious side effects, and quite frankly, it has been really easy for some people to forget about the vitamins they need, whether one or many.  With certain procedures such as gastric bypass, it becomes even easier to forget, especially with such small intake in general.  But you should keep in mind the following.  If you lack vitamin C, which is not common, but it still happens, you suffer from scurvy. 

When you suffer a deficiency of vitamin D, you get rickets.  Vitamin B1 causes beri beri, and vitamin B3 causes pellagra.  Vitamin B12 can cause pernicious anemia, and finally deficiencies of B6, B9, and B12 can cause serious birth defects.  In all reality, any vitamin deficiency could theoretically cause birth defects.  If you cook your food too much, you obviously lose nutrients, though cooking is not completely out of the question.  You can preserve your nutrients, and realistically the best way is to remember to put some amount of focus on fruits and vegetables in general.  But if you are missing any major vitamins, it can easily cause some serious side effects to be considered in any evaluation of health.