I was just wandering if Vault can call the backend storage asynchronously
Nope, Vault calls all of the storage back ends synchronously. It would take further API design to support async. We’d probably need to return something like an operation ID, and then allow look ups by storage ID to see if operations finished and succeeded, and/or provide callbacks. Also it would probably take some careful thought about design so folks didn’t find their stored data in unexpected states.