I don't know if this issue still exists in the latest vSphere version however there was always pretty easy work around my customer is using since then.
You can use Run Once commands in "Customization Template". Below, in vSphere Web Client screen shot, you can see just example where two MS-DOS commands (dir and echo) are used.
- C:\Windows\System32\slmgr.vbs -ipk H7Y93-12345-54321-ABCDE (just example use your product key)
- C:\Windows\System32\slmgr.vbs -ipk