These are handheld weapons, shaped like a pistol. They attach to the leg of a Vanir armored suit when not in use. They are capable of firing yellowish energy bolts that stun a Human in one shot, similar to a Wraith handblaster.
Two of the Vanir used those weapons to stun Doctors Daniel Jackson and Rodney McKay when they infiltrated Janus' secret lab in Atlantis in order to steal a component of the Attero device. Later, when Rodney McKay attempted to deactivate the device, the weapons were used to stun him, preventing him from accomplishing the act. After McKay and Jackson escaped and commandeered the suits, they stunned some of the Vanir guarding the Attero device. (SGA: "First Contact", "The Lost Tribe")