Since Vajrayana and Hindu Tantricism both use mantra and deities in their practices, many people confuse this fact into thinking that their practices are basically the same. This is, however, based only on the surface understanding of Hindu Tantricism and Buddhist Vajrayana.
The very base, i.e., the foundation on which Hindu Tantric practices is based, is to realise the Ultimate Real / Existent (Paramartha Satta). The foundation of the Buddhist Tantra, whereas, is to realise no Paramartha Satta. Anybody can see that to realise these two diametrically opposed bases would require an almost equally diametrically opposed path.
Just because both happen to use mantras and deities, it is naive to state that their practices are the same or even similar. To Buddhism, all other practices (meditation, mantra-visualization), no matter what name you give them, are perform to actualize some ‘thing’, be that for some material gain or for some subtle form like the Braman/Atma of Vedanta, or the Shiva-Shakti of Shaiva, and Shakta Tantra. They are only the extended versions of materialism. All of them are geared towards the achievement of one thing or the other, be it gross or subtle.
The strategy to free oneself from this sorrowful world by creating/fabricating (Parikalpit) on Absolute/Really Existing/Eternal Unchanging is merely a subtle version of the strategy of escaping from the problems and boredom of life with the help of money or other such things. All of these are escapist strategies and are thus labelled materialistic solutions. In Buddhism, spiritual materialism (Adhyatmic Bhautikavada) isthe search for the Eternal Unchanging Atman/ Shiva Shakti, and escaping to them from the sorrows of life.
The only true freedom is facing the actual situation of the world as it is (Tathata),without creating escapist dreams or fabricating dream realities unlike the world, like believing in unchanging, really existing thing.
The whole of the Buddhist path is geared towards teaching or re-learning to face the actual reality (Yathabhuta) and not to see the world according to a conditioned vision, whether they are ordinary human conditioning or conditionings learned through religions, for example - an Eternal Atman of the Upanishadas. According to Buddhism, the whole Hindu Tantricism is geared towards the realisation of a fabricated (albeit refined) dream like Atman.
Here, it is important to notice that it is possible that Hindu Tantricism, which is based entirely in search of an Ultimate Reality separate from this Samsara, can have the same practices as Buddhism. Hindu Tantric practices are based on the belief that the ten Mahavidyas (Ten great wisdom deities) really exist and by continuous Japa of their Mantra, one will slowly get their grace through which one will slowly identify with them, become the Mahavidyas, and be liberated. The progress of the practice starts from Dasoham (I am slave), where the practitioner believes in the deity as his/her Master or Lord. With continuous Japa of the deity (as the saying goes, ‘ japad siddhi, japad siddhi, japad siddhi nasamasaya ’ meaning, siddhi is attained through Japa .... no doubt ) one slowly merges into the deity (like Kali, Tara or Tripurasundari ) and becomes one with the deity. At this stage, it is called Soham ( That I am i.e., I am Shiva/Kali etc.)
Then, with more Japa, or more accurately, more Japa of the Mantras in series called Karma Dikcha, he/she becomes completely dissolved into Kali/Tripurasundari so that there is no ‘I’ and only the deity is left. This stage is called Naham (no me), and this is the Acme of Hindu Tantra where the personality has completely dissolved into one of the ten (dasa Mahavidya) and what is left is the ultimate reality called by whatever name - Kali, Dhumavati, so on.
In Mahanirvana Tantra, it’s written: ‘Karma dikcha yuttor devi, kramat sambhur bhavet’ (As the person becomes endowed with Krama Dikcha - serial mantra initiation, gradually he becomes Shival.
The actual modus operandi begins when the disciple first receives the initiation of one of the Mahavidya from a Guru. He learns the Mantra and Dhyana (visualization). He then does Japa Dhyana of the deity, i.e., he visualizes the deity in front of him and repeats the Nyasa and Mantra. Nyasa is placing the deities in different parts of the body and varies with different Kramas. So one imagines various deities (who are part of the Krama of the Mula Devata, main deity) in different joints of the body with one’s fingers and repeats the Mantra. Exactly how it is done differs with the various Kramas. The rest depends on doing more and more Japa (as said above) and completely getting oneself absorbed into the visualized deity until their unity. Finally, only the deity is left.
In Buddhist language, this is more and more Samatha (absorption) until identification and loss of self occurs.
It has to be mentioned that all forms of Hindu Tantra are not unanimous in their basic concepts, unlike all forms of Buddhist Tantra (who are all unanimous in their basic concepts).
1. Some forms of Hindu Tantra ( Kashmir Shaivism) believe that Shiva is in one’s own mind, but the majority believe that deities exist independently and the personality which is unreal dissolves into the Real Deity.
All forms of Buddhist Tantra believe the essence of all deities is one’s own mind.
2. Whereas visualization and Mantras in Hindu Tantra are limited to the ten Manavidyas as the highest forms of deities, but in Buddhist Tantras these are relegated to the positions o only protectors of the Dharma (Dharamapalas) who can only clear obscurations in a practitioner’s practice but not really give Enlightenment.
But besides these Dharamapalas like Mahakala or Mahakali, Buddhist Tantras also have visualizations and Mantras of Gurus, Bodhisattvas like Manjushree, Avalokiteswor, Vajrapani and Ista Devas. These, it is made clear especially the Ista Devas (called Yidam in Tibetan ) are your own Mind and not something separate. And it is only the proper use i.e. Samatha - Vipassyana of Mind Deities (the word Yidam in Tibetan means Mind Bond) that can liberate. Simply repeating Mantras and visualizing is not only said not to liberate automatically, but can also lead to more subtle forms of Spiritual Materialism according to Buddhist Tantra.