The Vanir are one of two groups of Gods in the Germanic pantheon (the other being Aesir), originating from the world of Vanaheim, and commonly holding domain over nature, fertility, and magic. 

The Vanir are only mentioned in Old Norse primary sources and not the broader corpus of continental Germanic or Anglo-Saxon lore.  This has led some scholars to speculate the idea of an Aesir-Vanir division is a later literary invention.  To modern Vanic Pagans, it cannot be disputed that the Vanir do have different spheres of influence than the Aesir, and indeed "feel" very different with regards to energy, the way They present Themselves, and Their agenda and workings in the world.  The Aesir-Vanir war as recounted in the Voluspa only says that three of the Vanir joined the Aesir - it does not say They ceased to be what They are.

Regarding the origins of the Vanir, and the implications of the Aesir-Vanir war, there seems to be two schools of thought among modern Vanic Pagans.

The first school of thought is that the Aesir and Vanir are two tribes within the Germanic pantheon, and the three ruling Vanir joining the Aesir represented a synthesis of parts into a greater whole.

The second school of thought is "the Vanir theory".  This theory is based on archaeological evidence denoting a pan-northern-European set of fertility Gods, with specific customs that were later associated with Gods counted among the "Vanir" (e.g. Frey, Nerthus).  The theory concludes that the Gods we know as the Vanir are pre-Indo-European fertility Gods, and the Aesir originated with the migrating Indo-Europeans coming from the Caucasus, and thus not are not from "the same pantheon"; that the story of the Aesir-Vanir war and subsequent merging of "Aesic" and "Vanic" traditions was based in the nomadic warriors conquering and intermarrying with the indigenous agriculturalists.  A more thorough explanation of this theory can be found here.

Because praxis follows theology, those whose beliefs are more in line with the first theory tend to identify themselves as Vanatru, and practice the more Aesic-Vanic blended religion of Heathenry reconstructed on the practices of the conversion era; those whose beliefs are more in line with the second theory may also identify as Vanatru, but some have begun exploring the new tradition of Waincraft.

As far as the Vanir Gods and Goddesses Themselves, according to the primary sources, there are three explicitly named Vanir: Frey, Freya, and Njord.  Snorri mentions Njord's sister-wife, who many scholars and Heathens agree is Nerthus, a Goddess mentioned by Tacitus.  Kvasir is another Deity named as Vanir, formed from the spittle of the Aesir-Vanir truce.  Of the three primary named Vanir, the names Frey and Freya mean "Lord" and "Lady" respectively, and the name "Njord" is of uncertain meaning.  Frey has domain over sunshine, rainfall, and the changing seasons.  He is associated with farming, and was called upon at weddings.  Njord, His father, is associated with fishing, and is noted for wisdom.  Freya is a Goddess of both love and war, as well as magic.  Both Frey and Freya were often depicted in antiquity in an explicitly sexual way.

It may be that there are a multitude of unnamed inhabitants of Vanaheim who are Powers of nature but largely unconcerned with interacting with humanity at this point in time.  However, many who identify as Vanic Pagans, through esoteric work, have discovered there are Gods mentioned in the primary sources who fit the Vanic type very well, and more so for having a vague reference to lineage or none at all.

Finally, a note should be made with regards to the elves.  Frey is said to rule over the light-elves, who seem to be similar to but not the same as the Vanir.  The Vanir and the elves are probably related in the same way that the Aesir and giants are related; most Aesir are of giant lineage and the thing that differentiates between a God and a giant seems to be function and alignment, not lineage.  We can by comparison assume that the Vanir as a whole are Powers of the "really big" things of nature - birth and death cycles of entire ecosystems, weather patterns - and the elves are helpers for "smaller" things - plant growth, guardians of a specific bio-region.

Further reading:

• Deities Speculated to Be Vanir
• The Differences Between Aesir and Vanir
• Vanic Deity Correspondences
• Vanic Symbols