The Mahabharata gives the eight the only narrative they have, and it uses them to solve a problem of its own. The Vasus offend a sage by taking his cow, and they are cursed to be born as human beings. They arrange with the river goddess Ganga that she will bear them and release them from the human condition immediately, and seven of them are duly born and put into the water at once. The eighth, whose share of the offence was the largest, has to stay. He grows up as a man, and he is Bhishma, the grandfather of the epic, who will renounce a kingdom and a marriage and hold a promise for a lifetime and die on a bed of arrows.
The story is doing work in both directions. It gives a class of Vedic gods a career they never had in the hymns, and it gives the epic's most morally weighted minor character a reason to be the way he is: a divine being serving a sentence, whose extraordinary self denial is a condition rather than an achievement. Readers who find Bhishma unaccountable are usually reading him without this.
It also fixes the Vasus in a way the ritual literature never managed. After the Mahabharata, the eight are the ones from the story, and the disagreements between the older lists become a technicality that only the lists themselves preserve. The jyotisha tradition, giving them the mansion Dhanishta, inherits the epic version, so a system built on the Vedic sky reads the class through a narrative that the Vedic material would not have recognised.