This Gist is a reference on how to use Terraform's null_resource to apply configuration to Vault when the configuration endpoint doesn't define all of the expected http methods. (GET, PUT, POST, DELETE)

NullResourceToVault.tf

resource "null_resource" "secret_tune" {
  triggers {
    config_content = "${sha512("${file("data/sys/mounts/secret/tune.json")}")}"
  }
  provisioner "local-exec" {
    command = "curl -X POST -H \"X-VAULT-TOKEN:$VAULT_TOKEN\" -d '${file("data/sys/mounts/secret/tune.json")}' $VAULT_ADDR/v1/sys/mounts/secret/tune"
  }
}