The Aesir and the Vanir had been at war for a long time when they decided to make peace. To ensure this peace they traded hostages: the Vanir sent their most renowned gods, the wealthy Njord and his children Freya and Freyr. In exchange the Aesir sent Honir, a big, handsome man who they claimed was suited to rule. He was accompanied by Mimir, the wisest man of the Aesir and in return the Vanir sent their wisest man Kvasir.
Honir however, was not as smart as the Aesir claimed he was and it Mimir who gave him advice. The Vanir grew suspicious of the answers Honir gave when Mimir was not around. Eventually they figured out that they had been cheated and they cut Mimir's head off and sent it back to the Aesir. Fortunately, this betrayal did not lead to another war and all the gods of the Vanir were subsequently integrated with the Aesir. There is not much known about the Vanir of the time before the assimilation.