Is Tantrayana the same as Vajrayana? I'm reading a little bit from the Tibetan Buddhist lama Dhardo Rimpoche at the moment. He keeps referring to Tantrayana not Vajrayana when discussing Buddhist philosophy and practice. It's pretty obvious from context that this is the diamond vehicle i.e. Vajrayana. However I'm not clear whether the two terms are the same and interchangeable. Would Tantrayana refer to non Tibetan Vajrayana schools such as Shingon? Is Tantrayana  a subset of Vajrayana or is it a term only used by certain schools? Do they have a different nuances of meaning or are they genuinely exactly the same?
Vajrayāna (Sanskrit: वज्रयान, Bengali:
বজ্রযান)

is also known as Tantric Buddhism, Tantrayāna, Mantrayāna, Secret Mantra, Esoteric Buddhism and the Diamond Way or Thunderbolt Way. The Lama and the Guru yoga are central in this system.[1] Vajrayāna is a complex and multifaceted system of Buddhist thought and practice which evolved over several centuries.

Although known with different names they are slightly different according to their practices.
There are many ways in which Vajrayana can be divided or classified and one way is to differentiate according to schools

..there is historical evidence for Vajrayana Buddhism in Southeast Asia and elsewhere (see History of Vajrayana above), today the Vajrayana exists primarily in the form of the two major sub-schools of Tibetan Buddhism and Esoteric Buddhism in Japan known as Shingon, with a handful of minor subschools utilising lesser amounts of esoteric or tantric materials.

The Tibetan schools are:

Tibetan Buddhism
Nelpalese Newar Buddhism
Ari Buddhism
Azhali region

Japanese schools are:

Shingon Buddhism
Tendai Buddhism
Shugendo

etc...
History of Vajrayāna:

Vajrayāna probably came into existence in the 6th or 7th century, while the term Vajrayāna itself first appeared in the 8th century. The Vajrayāna was preceded by the Mantrayāna, and then followed by the Sahajayāna (*8th century - long haired Sahaja(founder of Mahamudra)-siddhis) and Kalacakrayāna (*10th century - which includes messianism and astrology).

Fourfold division in Tibetan Buddhism

The best-known classification, by the Gelug, Sakya, and Kagyu schools, the so-called Sarma or New Translation schools of Tibetan Buddhism, is to divide the Tantras into four hierarchical categories:

-Kriyayoga, action tantra, which emphasizes ritual;
-Charyayoga, performance tantra, which emphasizes meditation;
-Yogatantra, yoga tantra;
-Anuttarayogatantra, highest yoga tantra, which is further divided into &quot;mother&quot;, &quot;father&quot; and &quot;non-dual&quot; tantras.

Shingon and Tibetan Buddhism

The primary difference between Shingon and Tibetan Buddhism is that there is no Inner Tantra or Anuttarayoga Tantra in Shingon. Shingon has what corresponds to the Kriyā, Caryā, and Yoga classes of tantras in Tibetan Buddhism. The Tibetan system of classifying tantras into four classes is not used in Shingon.
Anuttarayoga Tantras such as the Yamantaka Tantra, Hevajra Tantra, Mahamaya Tantra, Cakrasaṃvara Tantra, and the Kalachakra Tantra were developed at a later period of Esoteric Buddhism and are not used in Shingon.

So Tantrayana as far as I know is another name for Vajrayana which is different from Shingon and other Chinese Esoteric Buddhism. Tantrayana or Vajrayana is the latest 10th century tradition which includes the highest yoga tantra (Anuttarayogatantra).