Vishnu stands by Indra, and Vritra gives up the three Vedas
Before the thunderbolt falls, Indra asks Vishnu to stand by him. Each time Indra raises the bolt Vritra buys time by surrendering a part of the sacred knowledge held inside him.
Of old everything was within Vritra: the Rik, the Yajus and the Saman. Indra, wishing to hurl the thunderbolt, asks Vishnu to stand by him and Vishnu agrees. Three times Indra aims, and each time the frightened Vritra gives up a store of strength to save himself: the Yajus formulas, the Rik verses, the Saman chants. This is why the three Vedas are used in that order and why a cake is offered to Indra and Vishnu together.
Indra raises his bolt at Vritra three times. Each time Vritra pleads and bestows his strength on Indra in the form of the Ukthya cup. At the third raising Vishnu supports Indra and tells him to strike. Having lost his magic power, which was the sacrifice, Vritra is killed; Vishnu is given a share in the cup because he said strike.
What is different here:
The thing surrendered is the Ukthya cup, not the Vedas; Vishnu speaks.
Sources: The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41); The Veda of the Black Yajus School (Taittiriya Sanhita), tr. A. B. Keith (1914)