Members of the Vanir are physiologically very similar to the Asgard, as they were descended from the same race. Like their Ida cousins, the Vanir employ cloning as a means of propagating their race. However, the Vanir have two small implants placed on their bodies, one located above the right eye and another on the left chest. It is unknown what purpose these devices serve, although it may be related to their more advanced cloning techniques.
Like the Asgard of the Ida galaxy, the Vanir suffered the negative effects of generations of cloning and re-cloning themselves and sought means by which to stabilize their genetic structure. However, where the Ida Asgard were unwilling to engage in acts that risked other lives in that endeavor, the Vanir, consumed with the desire to survive, believed that the ends justified the means. Their primary goal was survival, as they did not wish to end up like their distant cousins, which would result in the loss of a hundred thousand years of history.
With this objective, the Vanir began conducting experiments, similar to those conducted by Loki in the Milky Way galaxy, on Humans of the Pegasus galaxy, allowing them a level of success in stabilizing their genome and preventing extreme cellular degradation.
In order to survive on their Toxic planet in the Pegasus galaxy and fight off enemies, the Vanir made use of powered exoskeletons that served as life support, as well as battle armor. (SGA: "First Contact", "The Lost Tribe")
The Vanir saw the Ascended Asgard Ran as their one hope to continue their race. As Ran had Ascended before the degradation began, the Vanir believed that a combination of uncorrupted eggs and their damaged genetic material would allow them to create a new generation and continue their race. The Vanir Dis stated that they felt later generations could judge their immoral actions but they felt it necessary to ensure there would be later generations. (SGA: "The Third Path")
The history of the Vanir began over 10,000 years ago when they split from their Asgard kin and made their way to the Pegasus galaxy. This was during the early years of the war between the Lanteans and the Wraith. This distraction allowed the self-exiled Asgard to begin their experimentation on Humans in order to solve their Cloning problem without interference from either the Asgard High Council or the Lantean Council. They believed that the war between the Lanteans and Wraith would give them enough time to find a solution to the genetic problems caused by the repeated cloning of their species. However, the war took an unexpected turn when the Lanteans (who the Vanir believed would win due to their advanced technology) were forced back to Atlantis and abandoned the Pegasus Galaxy, leaving the Wraith to become the dominant species of the galaxy.
Unlike the Lanteans, who would have censured only their illicit research, the Wraith did not tolerate any species with advanced technology other than their own. To that end, they began attacking the Vanir. Caught off guard, what intergalactic starships the Vanir possessed were destroyed, and they lacked the means to construct new ones. In order to survive, they abandoned their settlements and retreated to a planet whose atmosphere was toxic so that the Wraith would not be able to find them.
Their new homeworld was somewhat tolerable to live on at first, only requiring the use of breathing apparatuses. Over the eons, the planet became more and more hostile, which required the Vanir to make use of powerful armored exoskeletons to protect them. This continued for nearly 10,000 years until the planet became too hostile for the race to survive on by the year 2008.
Desperate, they ventured out into the galaxy for the first time in centuries and discovered a secret outpost with a powerful weapon within it. They theorized that the device could cripple the Wraith on an ongoing basis. However, a crucial component was missing.
When Dr. Meredith Rodney McKay and Dr. Daniel Jackson found a hidden lab in Atlantis, they unwittingly transmitted a subspace signal that was picked up by the Vanir, who deduced that whoever sent the signal had found the means to make the weapon work. Three Vanir went to the city, obtained the missing component, and took McKay and Jackson with them to get the device up and running, something the Vanir were unable to do, due to their lack of the Ancient Technology Activation gene.
While holding the two captive, one of the Vanir revealed his identity to Jackson, as well as the history of his group. He stated that that it was a matter of survival that they use the Attero device, despite the fact that it caused Stargates to overload and explode while it was in operation. The Vanir traveled only by ship and were not concerned about the loss either of the gates or of anyone who might be nearby, only caring that it would help destroy the Wraith.
When Lt. Colonel John Sheppard arrived in a Traveler ship to rescue McKay and Jackson and stop the device, three Vanir spaceships engaged the Traveler ship in battle over the planet. The Traveler ship destroyed two, but the third one fled into hyperspace with the Attero device's control key.
In 2010, the Vanir began a new plan to save their race: force Ascended Asgard Ran to return to mortal form, so they could get at her eggs. As Ran Ascended before the degradation of the Asgard race began, her genetic code was undamaged and she had thousands of eggs the Vanir planned to use to save their race by combining the eggs with their own damaged genetic material, believing that they could start a new generation of their race this way. While they had a device that would force Ran back to mortal form, the Vanir feared that doing so or simply returning to mortal form would damage Ran's eggs and chased Doctor Elizabeth Weir who Ran had Ascended and returned to mortal form for experimentation to determine this possibility. During the chase, a Vanir spaceship was shot down over Sateda by Doctor Rodney McKay with a Puddle Jumper, only leaving one badly injured Vanir, Dis, alive.
Desperate to save Dis for answers, the Atlantis team took him to a planet where Traveler Doctor Dekaas had noted similar architecture to the crashed Vanir ship. On the planet, the team found an abandoned Vanir installation and used its field medical station to heal Dis' injuries. Dis explained the Vanir plan to the team, showing no remorse for the lengths the Vanir were willing to go to save their race as opposed to the Asgard who were willing to destroy themselves instead. Dis offered to stop chasing Weir in exchange for his freedom, but refused to cease the Vanir pursuit of Ran. Weir offered another solution however: approach Ran directly and ask for her help. Dis agreed to try this idea and the team and Dis used an old Asgard scout ship held in mothballs in the facility to travel to a shrine on Earth where Dis begged for Ran's help. Ran returned to mortal form and agreed to help the Vanir attempt to save their race. However, she warned their plan might not work and if it did, the Vanir might not get the result they wanted. Dis took it as it was the best hope the Vanir had in a long time. As they departed, Ran promised the Atlantis team that the Vanir would no longer be a problem to them. (SGA: "Unascended", "The Third Path")
The Vanir had advanced technology, vastly superior to anything the Tau'ri have developed themselves, ranging from hyperspace capable starships that could pass through Atlantis' shield with ease (although it was subsequently reconfigured to prevent this), to personal energy shields and transportation devices. They also possessed energy weapons similar to Wraith stunners and appeared to have at least a rudimentary understanding of Ancient technology, as witnessed by their discovery of the location and purpose of the Attero device.
It is unknown how much if any of their present technology was based on Asgard technology developed by their Ida galaxy counterparts, as much of their original technology, such as their intergalactic starships, were destroyed during their first war with the Wraith, which forced them to remain in the Pegasus galaxy. It's quite safe to assume they possess a heightened intellect like their cousins, so it's not surprising they have developed such advanced technology, even under the harsh conditions they live in.
One impressive piece of Vanir technology was their impregnable armored exoskeletons. The armor had a wide range of technological abilities imparted on its user giving them superiority in personal combat.
The weapons and shields used on Vanir ships appeared to be inferior to those of the Asgard in the Ida galaxy and the Lanteans, but they were a match for those of the Travelers. They originally had ships capable of intergalactic travel, suggesting that mainstream Asgard technology was more advanced in every aspect to the interstellar ships they now use. Indeed, they stated that they have no resources to build more intergalactic ships so their current ships inferior state is likely due to the lack of resources available for them to build better ships. Indeed, Doctor Rodney McKay was able to shoot down one of these ships with just a Puddle Jumper without intending to do so. (SGA: "Unascended", "The Third Path")
Additionally, the Vanir possess a superior cloning technique due to experiments on the Humans of the Pegasus Galaxy and although far from solving the problem, it leaves them slightly healthier than their Ida cousins. However, they have still taken great genetic damage and their bodies aren't going to last much longer either. (SGA: "First Contact", "The Lost Tribe", "The Third Path")
While they lost their major intergalactic ships in the first battle with the Wraith, the Vanir have an old Asgard scout ship in mothballs in an abandoned facility capable of making intergalactic journeys. Given the facility the ship was located in appeared to have been abandoned for thousands of years, it is very likely the Vanir lacked knowledge of the ship's continued existence. This scout ship could transition to hyperspace so smoothly it wasn't noticeable and could make the trip from Pegasus to Earth in around fifty hours, something that took Earth ships weeks to do. (SGA: "The Third Path")
They also possessed a device that could force an Ascended being back into mortal form, but they did not use the device since they feared it could potentially damage the eggs of the Ascended Asgard Female Ran, which would contrast with their plans to use her eggs to save their species.
Behind the scenes
Though not given a name in the two episodes they appeared in, the Vanir were originally named after the episode "The Lost Tribe" and received the name Lost Tribe. Later, in an interview with Martin Gero, GateWorld suggested the name Vanir which Martin Gero accepted as their official name. Vanir were the second group of gods in Norse mythology, less well known than the Aesir who lived in Asgard, due to the Vanir being more elemental forces and spirits than divinities.