In the Anushasana Parva of the Mahabharata, just like in the Shanti Parva, Bhishma gives advice to Yudishthira and concerning how to be a good king and how to be good person, while he is lying on a bed of arrows after the end of the Kurukshetra war. In this chapter, Bhishma describes the various worlds people can attain in the afterlife, by telling the following story. There was once a pious Brahmana named Gautama, who raised an orphan elephant after its mother died. One day Indra decided to test Gautama by taking the form of a king, who seized the elephant and tried to take it away. Gautama protested, saying that whichever world the king would go to when he died, Gautama would follow him there and take back the elephant!
In the course of the dialogue, Gautama mentions the various worlds you can attain in the afterlife. Basically Gautama says something like "If you go to this world, I'll go there and get the elephant back." Then the king says "I will surely attain a higher region than that", so Gautama mentions an even higher world, and so on. The regions Gauatama mentions from lowest to highest are: the world of Yama god of death; Mandakini the region of Kubera god of wealth; the peak of Mount Meru; the forest of Narada; the land of the Uttarakurus; the world of Chandra the moon god; the world of Surya the sun god; the world of Varuna the ocean god; the world of Indra; the world of the Prajapatis; Goloka the world of cows; and Brahmaloka the world of Brahma. And then Gautama mentions a world even higher than those:
There where the foremost of Rathantaras is sung, where altars are strewn with the sacred Kusa blades, for the performance of Pundarika sacrifices, there where Soma-drinking Brahmanas go on vehicles drawn by excellent steeds, proceeding even thither I shall force thee to yield up this elephant.
Rathantara is the name of a hymn in the Sama Veda. But my question is, what is the world being described here? Is it a description Vishnu's abode of Vaikuntha? The reason I ask is that in the Vishnu Sahasranama, Vishnu is described as Trisama, or the one who is praised by three Sama hymns, and I think one of the three is Rathanthara. Are there any descriptions of Pundarika Yagnas (lotus sacrifices) being done in Vaikuntha?
P.S. Not to keep you in suspense, Gautama eventually realized the king was Indra, and then Indra took Gautama and the elephant to Devaloka.