First of all vitamins cannot replace food. In fact, vitamins cannot be assimilated without ingesting food. Hence why they should be taken with a meal. Vitamins regulate metabolism, help convert carbohydrates and fat into energy, and assist in forming tissue and bone.
And now, straight from the National Cancer Institute; Antioxidants are substances that may protect cells from the damage caused by unstable molecules known as free radicals. Free radical damage may lead to cancer. Antioxidants interact with and stabilize free radicals and may prevent some of the damage free radicals otherwise might cause. Examples of antioxidants include beta-carotene, lycopene, vitamins C, E, and A, and other substances.