To document how accessing the Cloud vCenter using a "short" name is not supported in VMC.
Symptoms: Customer is getting the error "[400] An error occurred while sending an authentication request to the vCenter Single Sign-On server - An error occurred when processing metadata during vCenter Single Sign-On setup: the service provider validation failed. Verify that the server URL is correct and is in FQDN format, or that the hostname is a trusted service provider alias" when attempting to setup "short" name/CNAME access on their 1.20+ SDDC vCenter.
Cause
In order to allow the use of vCenter short names, it requires modifying specific files within the VCSA (webclient.properties). As this will make a snowflake within the VMC SDDC Fleet, this is unsupported in VMC SDDCs.
Resolution
Only the Fully Qualified Domain Name (FQDN) is supported for use in accessing a VMC SDDC vCenter.
Additional Information
https://kb.vmware.com/s/article/71387 - this is how to resolve it OnPrem vCenter, but this would make a snowflake in the VMC Fleet, hence it is not supported.
Impact/Risks: Customer may confirm it had previously worked in 1.18 SDDCs, but it will break in 1.20 or later SDDC deployments.