GreytoWhite

From what I understand Guru Rinpoche (Oddiyana) and Amoghavajra (Sogdia) were both of Greco-Persian ethnicity but brought Vajrayana to Tibet and China respectively. Vajrayana was largely influenced by Kashmiri Shaivism and the shentong view of some schools is largely indistinguishable from Advaita Vedanta. There is so much that is similar, I'm not discounting Vajrayana - I'm a Dudjom Tersar practitioner.

Vajrayana is highly influenced not only by Shaivic tantra in Northern India but also the Greek and Zoroastrian practices of modern Afghanistan/Pakistan/Tajikistan. Lama Glenn Mullin has stated that the Nyingma school was brought in from primarily Persian texts that were translated into Tibetan. Bonpo's later iteration seems to have come from Tajikistan nearby. We also see proto-Vajrayana preserved in the Shingon school of Japanese Buddhism. Manichaeism seems to have been an influence as well with many calling Mani the Buddha of Light.