最后，我弄清楚出了什么问题:

Finally I have figured out what went wrong:

我的payload.json内容错误

my payload.json content was wrong

应该是这样的:

{
      "kubernetes_host": "https://kubernetes",
      "kubernetes_ca_cert": <kubectl exec to vault pod and cat  /var/run/secrets/kubernetes.io/serviceaccount/ca.crt, now make the cert one line by following this answer: https://***.com/a/14580203/2054147>
}

现在端点下方的状态良好，并返回了client_token的愿望

Now below endpoint is working fine and returning the desire client_token

curl --request POST --data @payload2.json http://127.0.0.1:8200/v1/auth/kubernetes/login

感谢@John帮助我弄清kubernetes_host的最初问题.

Thanks @John for helping me to figure out the initial issue with kubernetes_host.