Vanaheim is named only a handful of times in the entire corpus and never described. What the sources give instead is the war that ended with it: the Aesir and the Vanir fought, neither side won, and they exchanged hostages. Njord and his children Freyr and Freyja went to the Aesir; Hoenir and the wise Mimir went to the Vanir, who beheaded Mimir when they concluded Hoenir was useless without him. The head is preserved and Odin consults it.
The war is one of the most discussed passages in the field because it looks like a merger. Georges Dumezil read it as a mythological memory of two divine functions, sovereignty and war on one side, fertility and wealth on the other, being combined into a single pantheon, and that reading has shaped a century of scholarship. Others have read it as reflecting an actual encounter between religious communities, which the evidence does not support.
The practical residue is that the gods most associated with prosperity, harvest, sea and sexual love are all technically foreign hostages within their own pantheon. That is an unusual arrangement, and the sources treat it as settled without ever explaining what Vanaheim was like or whether anyone remained there.