The Three Vehicles: Vajrayana
The Vajrayana schools emphasize the importance of lineage. The Buddha transmitted his power of realization to his students and they in turn did the practices and attained realization and transmitted their realization to their students on down the line.
The analogy that is often taught is that in the Vajrayana one is told there is a precious gem in the mud and you just have to reach in and grab it where as in the other vehicles you clean off the mud as you dig down to the gem. That method takes a lot longer.
Attritbuting a special importance and relation to the Guru is a prerequisite to traveling the Vajrayana Path. We need a guide to help us along the way. Thus it takes a lot less time.
Although the Buddha taught the Vajrayana to a restricted group of suitable disciples later in his life, the Vajrayana cycle of teachings did not became popular until the sixth century C.E.
At that time, many of the great Mahayana masters of scholarship took up the Vajrayana path in their later years and left their scholarly and monastic establishments to practice Vajrayana tantras outside any institutional context.
Following the Vajrayana Path, enlightenment can be obtained in one lifetime.