We currently utilize Kickstart and SCCM to deploy our initial systems. Our Linux team hates having to install the gugent RPM to tell vRA that the kickstart process is complete only to immediately remove the RPM. Is there an alternative to this. We would love to make a REST call to vRA instead.
From a supportability option, I think that's your only choice. It's the manager that controls this provisioning and signaling process, and so it may not even be possible to duplicate with an API (or at least any of the publicly documented ones).
In our environment we have a message bus we can monitor and update. During the imaging process messages are delivered that give the status of the build. We also update our machine database with this same build information so we can check either spot for the completion. Both have api's. The message bus is more accurate time wise but either works for us. You could engage with those teams to see if they can make something that is easily queryable and then embed those in the api. Another option is using the guest tools, ssh, powershell, something that logs into the vm with a service account and can examine local logs for something similar.