Without the education of switching to a Vegetarian or Vegan diet, it may do more harm than good. Here are some resources on getting the proper vitamins in your diet on a vegetarian or vegan diet. The most important vitamins are B12, D, and sometimes Iron & Calcium.
B12 is vital for vegans and can be found in fortified soy, rice, or non-dairy milk, fortified breakfast cereals, tofu and prepared meatless meats, or in a supplement. You often can find iron and calcium in fortified non-dairy milks and cereals as well.