You can handle that in a few different ways.

I usually do something like an eval loop and try to hit the ESXi API (assuming it isn't in lockdown mode). Once the API is up, you can be pretty sure it can be added to vCenter.  Since you'll need the SSL thumbprint of the ESXi host anyway, usually you can just get that from ESXi API endpoint, or the web file service.

lamw explains how to do it with openssl here

I explain it here

I once looked at using the Net::SSLeay library (if you are comfortable with the CTX_* properties from the OpenSSL C lib) to do the thumbprint extract as well.

Basically, you'll loop-eval the call to the ESXi web service(s), generate the thumbprint, and add the host to vCenter.  I usually implement a method with a timeout-waitloop for the host reboot (e.g., 10m timeout & 30s retry intervals)