I don't know. Sometimes I wonder if maybe there will be an explosion of Vajrayana teachings on the tantra in the coming centuries if it's not too late for humanity. I wouldn't put my money on it. It's hard to believe. The deepest wisdom of Buddhism is secret in that it should only be revealed to people who are ready for it in furtherance of enlightenment and ultimate realization. Vajrayana is that secret wisdom. 

In Buddhism, there is Theravada (the foundational vehicle), Mahayana (the "great" vehicle) and Vajrayana – esoteric teachings, secret, ultimate. I want to be careful not to suggest that there is a progression or that either Theravada, Mahayana or Vajrayana Buddhism are better than one or another. Perhaps distinctions exist from a dualistic, analytic human level, but each vehicle is an all-encompassing, stand-alone practice. The distinctions exist for distinct types of people, personalities and aptitudes. 

What is now called Theravada Buddhism is what the Buddha directly taught his disciples; the basic principles of monastic living and how to pursue enlightenment and the underlying philosophies. It wasn't incomplete, it was just what was appropriate at the time to turn the dharma wheel to get people on the path towards enlightenment.

Mahayana was revealed some 500 years later with the codification of the Lotus Sutra. Well, not exactly 'codification', that's when it was reportedly first written down, but it had been passed down orally for centuries prior. The oldest extant manuscript I think is from much later in the 7th century C.E. It is claimed that what became the Lotus Sutra was based on the historical Buddha's final sermon, but it took 500 years for it to become understood and appreciated. 

The written Lotus Sutra is therefore metaphoric and enhanced, rather than literally what the Buddha actually taught. It includes text of what was likely orally transmitted in verse form, as well as prose versions expounding on the verse portions. Its current form is clearly a work of fiction, but the principles of what he taught are incorporated in it and are the basis of the Mahayana philosophy, which on top of the basic teachings of the Theravada, added the concepts of universal compassion and that enlightenment is the ultimate destiny of all living beings.

Theravada and Mahayana Buddhism are predominant in the western world today. Buddhist teachers strive to teach people Theravada and Mahayana principles, depending on preference, but they don't challenge people too much beyond their capacity to understand. They teach within the context of the reality people live their lives. Most people, including self-identified Buddhists, are just trying to live their lives. They want to be calm, they want to face challenges, they want to understand, but they're still attached to their ego and their lives and are unable to see reality beyond how it presents itself. They want good relations, they want to feel good.

That's all good and great. That's what much of Buddhist literature supports.

But recently Vajrayana has also found its way more and more into the discussion with a lot more literature being produced in the west. Vajrayana has been introduced via Tibetan Buddhism, which ironically has entered the western consciousness largely thanks to the Communist Chinese invasion of Tibet in 1959, sending scores of refugees to create a Tibetan diaspora with Buddhism as a spearhead. The Dalai Lama has as much acknowledged the irony of the Tibetan tragedy being instrumental in bringing Vajrayana Buddhism to the west. 

But Vajrayana is not different or separate from Mahayana. Zen is understood as Mahayana, but Zen also promulgates the same ideas as the Vajrayana. Zen is complete without Vajrayana because Vajrayana is nothing more than Mahayana beyond different and enhanced methodologies. Going deeply into Theravada, Vajrayana is also there and is therefore also complete in itself. 

All the vehicles of Buddhism aim towards an ultimate understanding of reality. Just recently I bought a book by Zen master John Daido Loori, "Teachings of the Earth", and although steeped in the Mahayana Zen tradition, he teaches an understanding of the ultimate reality which is the same as in Vajrayana. 

Vajrayana methodology is radical, though. The concepts presented may elicit many "wtf?" responses. It's a radical interpretation of reality that's not based on gross human senses and interpretation. Actually, forget about wondering if there might be an explosion of Vajrayana teachings in the coming centuries. And hope there won't be. Once you expose something to the masses, it all goes to shit. So it is writ.

ptd 3/24/19 -ed.