Background/Objectives: Traditionally classified as nutrients or micronutrients, vitamins can now be viewed as multi-functional excipients that have considerable potential within the context of pharmaceutical formulation science. Due to their physicochemical characteristics, antioxidant nature, and stabilizing action, vitamins are able to serve functions that go beyond their nutritional applications. These include but are not limited to solubility, increased permeation, controlled release, as well as synergistic stabilization and enhancement of the active ingredient(s). Results: The current work presents the synthesis of information available to date regarding the classification, mechanisms, and excipient properties of vitamins and their role in novel drug delivery systems development. In addition to the conventional classification into water-soluble and fat-soluble, some other classification criteria are used based on the chemical nature of vitamins. Water-soluble vitamins are mainly used to facilitate oxidation and pH adjustments whereas fat-soluble vitamins are mostly employed in lipid-based systems due to solubilizing and antioxidative effects. Conclusion: Vitamin-based mechanisms such as the surfactant effect of TPGS (vitamin E), oxidation and reduction reactions in vitamin C, or hydrotropy in niacinamide illustrate active involvement of vitamins in enhancing bioavailability and improving the formulation. Novel areas of application range from vitamin-functionalized nanocarriers to receptor-targeted ligands and incorporation into 3D-printed drug delivery platforms.