Vajrayana is the smallest of the three great streams, put at roughly 6 per cent of Buddhists in the estimates most often reproduced. No reliable standalone count exists for Tibetan Buddhism as such.
It is the state religion of Bhutan and the prevailing tradition of the Tibetan plateau, Mongolia, Nepal, Ladakh and the other Himalayan regions, of the Russian republics of Kalmykia, Buryatia and Tuva, and of parts of northeast China. Shingon in Japan and the Newar Buddhism of the Kathmandu valley are Vajrayana traditions outside the Tibetan orbit entirely.
The tradition's modern history is dominated by the Chinese occupation of Tibet from 1950 and the flight of the fourteenth Dalai Lama in 1959. Monasteries were destroyed in very large numbers during the Cultural Revolution and have been partly rebuilt under conditions the Chinese state controls. The exile community rebuilt its institutions in India and Nepal, and from there Tibetan Buddhism reached the West on a scale no other Buddhist tradition has matched in the same period. The succession to the Dalai Lama is an open political question: the Chinese government asserts a right to approve the recognition of reincarnations, and the exile administration rejects the claim.
Vajrayana in the West has also produced a run of documented abuses of the teacher-student relationship, in several of the largest convert organisations. The tradition's own texts warn at length about the choice of a teacher. They did not prevent it.