The Vanir are descendents of Njord, son of Buri. They lived in Vanaheim for tens of thousands of years. That land is close to the flow of magic of the Demiurge and the Vanir have produced mystics of astounding power and skill over the millennia.
The Vanir built towering skyscrapers and citadels, tall enough to be seen from deep space and visible from every part of the continent of Asgard. They focused deeply on magic and research into it, as well as becoming masters of prophecy and prognostication.
The Vanir were devastated by Ragnarok and counterattacks by alien races that followed. They abandoned their citadels to join the Asgardians in the center of the landmass, leaving behind ancient constructs filled with traps, magical artifacts, and secrets both grand and terrible.
Odin met with King Freyr after Ragnarok and tried to convince him that the Asgardians and Vanir should forge an alliance of marriage. The war was short lived but furious, furthur depleting the population of Asgard (and encouraged by Surtr, who sought to keep the city destroyed). In the end peace prevailed and Odin married Freyja. From them, Asgard became known as the home of the unified races-- The Aesir.