Short answers, pulled from the story.
Vajrayana translates as Diamond Vehicle or Thunderbolt Vehicle, drawn from the Sanskrit word vajra, which denoted both a mythical indestructible weapon and the ultimate nature of reality described in the tantras as pure and radiant yet unbreakable. Earlier names for the same tradition included Mantranaya (Path of Mantras) and Mantrayāna (Mantra Vehicle), which were the initial terms used in medieval India.
Vajrayana emerged between the 5th and 7th centuries CE in medieval India, developing out of pre-existing Tantric traditions that had already arisen within Hinduism during the first millennium CE. Distinct Buddhist tantric traditions crystallized by the 7th century CE and then spread rapidly across Southeast, East, and Central Asia.
Vajrayana claims to offer a faster path to Buddhahood than the older Paramitayana, which its scriptures say requires three incalculable aeons. Vajrayana literature states that the Mantra vehicle can lead to full enlightenment in a single lifetime. The key distinguishing method, identified by the Tibetan scholar Tsongkhapa, is deity yoga: meditating on a chosen deity while visualizing oneself as that deity, a practice absent from ordinary Sutra-based Buddhism.
The mahasiddhas were wandering tantric yogis in medieval North India who practiced methods radically different from those of Buddhist monasteries, including rituals on charnel grounds and tantric feasts at sacred sites. According to Robert Thurman, they thrived during the latter half of the first millennium CE. A movement called Sahaja-siddhi, according to Schumann, developed among them in the 8th century in Bengal, with long-haired wanderers openly challenging the monastic establishment.
Vajrayana reached Japan through Kukai, a Japanese monk who studied in China during the 9th century Tang dynasty and returned with Vajrayana scriptures, techniques, and mandalas. Kukai founded Shingon Buddhism, whose primary texts are the Mahavairocana Sutra and the Vajrasekhara Sutra. Shingon is one of the few surviving Buddhist traditions that still uses the Siddham script of Sanskrit.
The vajra is both a ritual implement and a symbol of the tantric path and its goal. As a symbol it represents the mind that has realized ultimate truth inseparably merged with a special kind of tantric bliss, described as the wisdom of nondual emptiness and bliss. In ritual, the vajra is used in combination with a bell: the vajra represents method and great bliss, while the bell represents wisdom realizing emptiness, and their union symbolizes the integration of compassion and insight.