The Sankalpa Suryodaya is an allegorical play composed by the Sri Vaishnava Acharya Vedanta Desikan. In this excerpt from the Sankalpa Suryodaya, a character representing Vedanta Desikan tells a character representing Ramanujacharya that he looks like Vishnu's incarnation Dattatreya:
Divine sire! In the ascetic garb thou seemest a Dattatreya described by the great sages as having defended the Vedas, Dattatreya who was Vishnu himself incarnate bearing the Tridanda, when inroads were made into them by Kanadas, Buddhas and Pashandas.
My question is, what is the story of Dattatreya defending the Vedas from the attacks of "Kanadas, Buddhas and Pashandas"? Presumably Kanadas refers to members of the Vaisheshika school, Buddhas refers to Buddhists, and Pashandas refer to Jains, Charvakas, etc. The Vaisheshika school is probably included because even though they accepted the Vedas, did not accept Sabda or scriptural authority as an independent Pramana as you can see in the chart in my question here.
Now as I discuss in this question, Dattatreya is an incarnation of Vishnu who is the son of the sage Atri and his wife Anusuya. Dattatreya is a great guru, as described in this chapter of the Srimad Bhagavatam:
The sixth incarnation of the puruṣa was the son of the sage Atri. He was born from the womb of Anasūyā, who prayed for an incarnation. He spoke on the subject of transcendence to Alarka, Prahlāda and others.
And another chapter of the Srimad Bhagavatam:
The great sage Atri prayed for offspring, and the Lord, being satisfied with him, promised to incarnate as Atri’s son, Dattātreya [Datta, the son of Atri]. And by the grace of the lotus feet of the Lord, many Yadus, Haihayas, etc., became so purified that they obtained both material and spiritual blessings.
And he is said to have taught Vishnu's incarnation Parashurama a text called the Tripura Rahasya. But does anyone know of an scriptures that mention him defending the authority of the Vedas.
EDIT: This excerpt from the Yatidharma Samucchaya, a work by the 11th century Sri Vaishnava Acharya Govinda Jeeyar, gives the following quote from some work composed by Dattatreya:
The law contained in the triple Veda was once assaulted by Buddhists, heretics, and the followers of Kanada, and it was protected by Vishnu carrying a triple staff.
This exactly matches what Vedanta Desikan says in the quote above, so Dattatreya may be referring to himself.