The only Vanir ever named in the surviving sources are a father and his son and daughter: Njord, and his children Freyr and Freya. The sources say plainly that Njord's children were born of his own sister, a marriage acceptable among the Vanir by their own custom, though the Prose Edda notes it was forbidden once the two families merged at Asgard. Scholars read that detail two ways: as a trace of a genuinely older layer of Germanic religion, organized around fertility and different kinship rules than the sky and war gods who came to dominate later, or as a literary device explaining why the two families needed a formal reconciliation at all.
Georges Dumezil's trifunctional hypothesis, which reads Indo-European pantheons as consistently organized around three functions, sovereignty and magic, physical force, and fertility and wealth, places the Vanir squarely in the third function against an Aesir who cover the first two. Not every Norse specialist accepts the comparison, but it has shaped most later discussion of why the Norse pantheon divides into two families at all rather than one.
The clearest evidence for an actual Vanir cult, rather than only a mythological family, is Adam of Bremen's eleventh century account of the temple at Gamla Uppsala, which places an image of Freyr, described as governing peace and pleasure and given an oversized emblem, beside those of Odin and Thor, and records a nine yearly sacrifice held there. The account is a secondhand description written a century after conversion by a cleric hostile to the religion, and should be weighed with the same caution given it elsewhere on this atlas, but it remains the strongest surviving link between the Vanir of myth and an actual place where they were worshipped.