Vamachara is not a synonym to tantra but one of the achara of tantra.
"Tanyate, vistaryate jñanam anena iti Tantram." According to this derivation of the word Tantra from the root "Tan" "to spread," it is defined as the Shastra, by which knowledge (Jñana) is spread. Mark the word Jñana. It is here we find that variety which is so puzzling to those who have not gone to the root of the religious life of India. The end is substantially one. The means to that end necessarily vary according to knowledge, capacity, and temperament. But here again we may analyze the means into two main divisions, namely, Vaidik and Tantrik, to which may be added a third or the mixed (Mishra). The one body of Hinduism reveals as it were, a double framework represented by the Vaidik and Tantrik Acharas, which have in certain instances been mingled.
According to the Kularnava and Jñanadipa Tantras there are seven Acaras of which the first four, Veda, Vaishnava, Shaiva and Dakshina belong to Pashvacara; then comes Vama, followed by Siddhanta, in which gradual approach is made to Kaulacara the reputed highest. Elsewhere six and nine Acaras are spoken of and different kinds of Bhavas, Sabhava, Vibhava and Dehabhava and so forth which are referred to in Bhavacudamani.
The Pañcatattva are either real (Pratyaksha. "Idealizing" statements to the contrary are, when not due to ignorance, false), substitutional (Anukalpa) or esoteric (Divyatattva). As regards the second, even a vegetarian would not object to "meat" which is in fact ginger, nor the abstainer to "wine" which is coconut water in a bell-metal vessel. As for the Esoteric Tattva they are not material articles or practices, but the symbols for Yogic processes. Again some notions and practices are more moderate and others extreme.
Thus we find in this Pañcatattva Ritual a counterpart to the Vaidik usage of wine and animal food. As regards wine, we have the partaking of Soma; meat was offered in Mamsashtaka Shraddha; fish in the Ashtakashraddha and Pretashraddha
Vamacara means literally "left" way, not "left-handed" in the English sense which means what is bad. As the name is given to these Sadhakas by themselves it is not likely that they would adopt a title which condemns them. What they mean is that this Acara is the opposite of Dakshinacara. Philosophically it is more monistic. It is said that even in the highest Siddhi of a Dakshinacari "there is always some One above him"; but the fruit of Vamacara and its subsequent and highest stages is that the Sadhaka "becomes the Emperor Himself". The Bhava differs, and the power of its method compared with Dakshinacara is said to be that between milk and wine.
Kauladharma is in no wise sectarian but on the contrary claims to be the head of all sects. It is said
"at heart a Shakta, outwardly a. Shaiva, in gatherings a Vaishnava
(who are wont to gather together for worship in praise of Hari) in
thus many a guise the Kaulas wander on earth."
Antah-shaktah bahih-shaivah sabhayam vaishnava matah
Nana-rupadharah Kaulah vicaranti mahitale.
A rudimentary form of Shakta beliefs and practices can presumably be traced to the ruins of the pre-Vedic Harappa civilization. The earlier Vedic tribes, whose material cultures and social institutions have been revealed in the Rig Veda, appear to have disliked the conception of the Female Principle owing to their patriarchal bias, but [even] they had to incorporate some female deities into their pantheon.
Shaktism and Agama worship in Devi Bhagvatam:
Thus the Durgâ Devî became very widely celebrated in this world. O king! Thus in different countries, the devotion began to increase towards the Goddess.
The Devî Bhagavatî Bhavânî became in every way an object to be worshipped and adored by all people and everywhere in Bhâratavarsa.
The people began to recite slowly, meditate, and chant hymns as advocated by the Âgamas constantly and became deeply attached to the S’akti worship and began to be looked upon with the highest honour by others.
O king! From that time all the people used to worship, perform Homa ceremony and sacrifice duly in honour of the Devî in every Navarâtri (for the first nine days of the bright half in the months of Âs’vin and Chaitra).
There was a revival of the pre-Vedic Mother Goddess cult in the post- Rigvedic age, probably due to the initiation of the Vedic tribes into the agricultural way of life and agricultural rituals; and since then, the Female Principle never ceased to be an important cult of the people. It was so deep-rooted in the Indian mind that even in sectarian religions like Vaishnavism, Shaivism, etc, the Female Principle had to be given a prominent position. Even the basically atheistic systems like Buddhism and Jainism could not avoid this popular influence. Later Buddhism is, in fact, nothing but a disguised Tantric cult of the Female Principle.
Each of the ten personalities of the Divine Mother in Tantra had been formulated based on certain concepts and realizations of the Vedic seers. Also, for certain deities, in addition to prescribing the Tantric Mantras, the Tantric Seers prescribed Vedic verses for worship. Some of the examples cited by Shankaranarayan are
In the Rig Veda, Agni is shown as having the power of carrying the spiritual aspirant across all tribulations while Aditi (Divine Mother) is said to afford shelter as a ship. In Tantra, these functions are built into the conception of **Tara**.
The Vedic symbolism of “primordial darkness covered by darkness” gave rise to the concept of **Dhumavati** (Divine Smoke Screen) in Tantra.
The Rig Vedic primal seed of mind, Desire (Kama) was amplified as **Kamakala**, the concept of **Tripurasundari**.
The all-pervasive indivisible infinity of the Divine Mother, Aditi, was used to build the concept of **Bhuvaneshwari** (she who pervades the worlds).
The mounting aspiration concentrated at the base of things, flaming forth as Agni became the principle of **Tripura Bhairavi**.
Kapali Sastry notes that in the Vedas, Agni is the seer who finds the way, who burns the dross and cleanses the seeker of all sin and impurity, carries him through all obstacles, like a boat over the seas. In the Shakta Tantra, this same Deity came to be worshipped as Durga, the indomitable, the protectrix who carries the devotee safe across the sea of misery, the ocean of birth and death. More importantly, he found that in one place the exact Mantra addressed to Agni in the Vedas had been applied in Tantra to Durga
The Vedic theory of Vak posits a Universe created by sound (cosmic vibrations) and it says that this Sound exists at four levels, from the subtle to the gross, in the Universe (see blog post Vedic Vak : four levels of sound). In his book Sidelights on Tantra, Kapali Sastry discloses how Tantra incorporated the Vedic theory of Vak and adapted it for practical purposes. Tantra correlated the four levels of Vak to the four subtle centers (Chakras) in the human subtle body (Para to the Muladhara Chakra; Pashyanti Vak to the Manipura Chakra in the navel; Madhyama to the heart; Vaikhari to the throat). In the Vedas, the beginning of creation is signified by the symbol of the “Bull that Roars” (Rig Veda 4.58.2). In Tantra, this concept is represented by the Adya Spanda (i.e. primal vibration). The original Word (AUM – primal root sound) of the Veda was further developed by Tantra into the theory of seed sounds (bija-aksara) which was then applied to the practice of Mantras.
There is also a connection between the Upanishadic Vidyas and the Tantric Goddesses. In his book Ten Great Cosmic Powers, Shankaranarayanan credits Sri Vashishtha Ganapati Muni, disciple of Ramana Maharshi, with revealing how the ten Personalities of the Divine Mother can be traced to the Vidyas in the Upanishads.
Tantra in Upanishad vidya:
Kali ---- Prana Vidya
Tara -----Akshara Vidya Brihadaranyaka Upanishad III. 8
Tripura Sundari ---- Vaishwanara Vidya
Bhuvaneshwari -----Parovariyasi Vidya of the Chandogya Upanishad 1.9.
Tripura Bhairavi -----Shandilya Vidya of Chandogya III.14
Chinnamasta ----Jyotirvidya of Brihadaranyaka Upanishad
Dhumavati -----Dhuma Vidya of Chandogya Upanishad
Bagalamukhi ----Indra Yoni Vidya of Taittiriya Upanishad
Matangi ----Udgitha Vidya
Kamalatmika Madhu Vidya of Brihadaranyaka Upanishad
Kulluka Bhatta, the celebrated commentator on Manu, says that Shruti is of two kinds, Vaidik and Tantrik.
Vaidiki tantrums caviar dvividha shrutih kirtita
What is there in the great Devi Sukta of the Rigveda (Mandala X, Sukta 125) which the Shakta Tantra does not teach? The Rishi of this revelation was a woman, the daughter of Rishi Ambhrina. It was fitting that a woman should proclaim the Divine Motherhood. Her Hymn says: "I am the Sovereign Queen the Treasury of all treasures; the chief of all objects of worship whose all-pervading Self all Devatas manifest; whose birthplace is in the midst of the causal waters; who breathing forth gives form to all created worlds and yet extends beyond them, so vast am I in greatness." (The full Hymn is translated in the French Edition of A. and E. Avalon's Hymns to the Goddess, Bossard, Paris.)
From the above points, we can see that shaktism was present since time immemorial and tantric practices are indeed part of ancient Indian tradition. Vamachara is a stage of tantra worship hence it is also a part of ancient agama based worship.