The Host part of the URL doesn't actually get used when we initiate connections to UNIX domain sockets. As of https://github.com/golang/go/issues/60374 (in the latest Go releases at the time of this writing), we must set it to something that looks like a hostname or requests will fail.
This provides the github.com/hashicorp/vault/api package which contains code useful for interacting with a Vault server.
For examples of how to use this module, see the vault-examples repo.
For a step-by-step walkthrough on using these client libraries, see the developer quickstart.