There are two main branches of Buddhism ... those that have the Bodhisattva tradition and those that do not.
Those have have the Boddhisattva tradition are called "Mahayana" Buddhism.
Tibetan Buddhism is ONE school of Buddhism that has the Boddhisattva tradition. Therefore it IS Mahayana Buddhism (Zen is also a well-known Mahayana school).
Sometimes they say there are 3 main branches: Theravadan, Mahayana, and Vajrayana.
Vajrayana is actually a "subtset" of Mahayana (being one of the Mahayana schools) and it refers to the particular type of Buddhism that the Mahayana Tibetan Buddhism practices.