Dunno if this is of any significance, but my bet is Vajra is a weapon.
Why?
The Vajra used to be a religious artifact, but from my understanding, when I read "Secrets of the Samurai - The Martial Arts of Feudal Japan" by Adele Westbrook and Oscar Ratti, they described the Vajra as more of a weapon, highly favoured by Buddhist monks. The Vajra itself underwent many modifications, not just aesthetic, but practicality, as it was used to deflect and parry weapons, especially snapping sword blades.
Having said that though, the writers could have just thought that it was a cool name to use, and might not relate to it being a weapon at all.