The mythological dualism, of Aesir and Vanir, inherent in Nordic mythos is a puzzling one. The orthodox account has it that the split represents the gods of two cultures, an ancient indigenous one, worshipping the Vanir, and 'new comers' who brought the Aesir mythos. But while there is no doubt some truth to this it is not a satisfactory explanation. This kind of cultural sedimentation occurs in all cultures, and usually results in mythic synthesis rather than dualism.
An alternative theory put forward by Nietzscheans has it that the Aesir-Vanir polarity represents a northern version of the Apollo-Dionysos polarity. This makes sense, the Vanir are wild forces of nature, worshipped communally, an essentially Dionysian mode of myth, while the Vanir are atomic personalities (often originating in ancestor figures), representing domains important to humans in a highly culturally specific way (often bearing laws) related to on a purely personal level, an Apollonian mode of myth. The figure of Kvasir produced by their fusion, is remarkably like Orpheus, the great mediator between the Dionysian and the Apollonian, his Aesir equivalent Mimir, even loses his head in the same way as the Greek seer.
This thesis is very convincing, though it has some problems, for one, Woden last ruler of the Aesir, whose name means 'fury', is very Dionysian (as are some other Aesir), also many Aesir represent impersonal forces of nature. Secondly, the Vanir are just as Germanically culturalised as the Aesir and share many features with them. So the distinction is not entirely clear. However given the power of the Nietzschean interpretation these problems might be explainable in terms of later mythic synthesis on top of the dualism (perhaps after the significance was lost). On the other hand any synthesis does appear to be quite ancient. Woden for instance is held by some to be a fusion of the (Vaniric) storm giant Wode, with an ancestorial shaman archetype, who later merged with the Aesiric Sky Father and his cultural order.
Then again perhaps the originators of the mythos were more sophisticated than commonly thought and recognised that every pole of a duality contains its opposite. We shall never know.