Whether Vajrayana is a separate vehicle at all is a live classification dispute, and this entry has to start there, because the answer changes what the subject is.
The third-vehicle model treats Vajrayana as standing alongside the Sravakayana of the early schools and the Mahayana: a distinct yana with its own scriptures, its own initiations and its own account of the goal. That is broadly how the Tibetan traditions and the Japanese Shingon school present themselves. The branch model treats it instead as a set of methods within Mahayana. The goal is identical, buddhahood for the sake of all beings, and what differs is the means and the claimed speed. Much of the scholarly literature takes this second line, and the traditions themselves supply the reason it is defensible: the classic claim made for tantric practice is that it delivers in one lifetime what the ordinary Mahayana path takes three incalculable aeons to reach. That is an assertion about method, not about destination.
Both models are in serious use. This atlas gives Vajrayana its own entry because its texts, institutions and practices are separable in a way that makes the material easier to describe, not because the third-vehicle claim has won the argument.
What is not in dispute is the material. Tantric texts and methods appear in India between roughly the fifth and seventh centuries, drawing on currents shared with the Hindu tantra of the same period, and dating the individual tantras is described in the field as a task that is not merely difficult but impossible.