Trying to add support for interacting with Vault agent: jenkinsci/hashicorp-vault-plugin#36
Found out after much headache my issue was with vault-java-driver because it assumes vault-token is mandatory, which it is not in the case of the auto-auth use case for Vault agent. The null pointer results in 400 Bad Request.

Example of vault cli against the very same endpoint (ngrok used for inspection purpose 😓)

$ VAULT_ADDR=http://0c19b983.ngrok.io vault kv get kv-v1/admin
==== Data ====
Key     Value
---     -----
key1    123
key2    456

You can set up the vault agent to use auto-auth and handle the requesting the token. So the application does not need to be concerned about caching or reauthenticating against vault.

https://www.vaultproject.io/docs/agent/