Vasu is an ordinary adjective before it is a divine name. It means good, or bright, or excellent, and as a noun it means goods, wealth, the things worth having. In the Rigveda the Vasus are a group rather than a set of individuals, generally named under Indra, and it is often impossible to tell whether a given verse means a class of gods or is simply calling some other gods good ones. They are one of the standard triads of the older material, invoked with the Adityas and the Rudras, and in that formula they occupy the position of the beneficent.
The number eight and the membership are later, and the membership never quite steadied. The lists that survive give something like earth, water, fire, wind, the fixed pole star, the moon, light and dawn, wearing names that are transparent to those meanings, and they differ in which of them are included and in what order. That variation is not damage. It is what happens when a tradition inherits a class with a fixed size and an unfixed roll, and successive compilers fill the eight slots from a template of the natural world, each choosing a slightly different eight.
What the Vasus are gods of, on any of the lists, is the furniture of the habitable world: the elements and the abodes, the things that make a place somewhere one can live. That reading fits the name. A group called the good ones, filled with earth and water and fire and the light, is a way of saying that the world is provisioned, which is a different religious claim from the ones the Adityas and the Rudras carry, and the triad works because the three are doing three different jobs.