Definitive radiation therapy is the backbone of treatment in the management of primary vaginal cancer. The use of modern treatment techniques, such as intensity-modulated radiotherapy (RT) and image-guided adaptive brachytherapy, allows for the precise delivery of higher radiation doses to the tumor with reduced exposure to surrounding normal tissues compared with older, conventional 2D techniques. These advances in treatment planning have resulted in a nearly 10% absolute improvement in survival, along with a significant reduction in severe treatment-related toxicity. Accurate target volume delineation based on the tumor location and extent at presentation is crucial and is achieved with greater precision due to advances in imaging, including the routine incorporation of MRI into treatment planning. The brachytherapy target volume concepts used in the management of vaginal cancers largely mirror the adaptive approaches established for cervical cancers, incorporating tumor response following external-beam RT. Additionally, the expanding array of brachytherapy applicators, including both hybrid and custom options, has improved the ease and conformality of tumor implantation compared with standard template-based approaches. This review summarizes modern RT concepts and techniques in the management of primary vaginal cancer.