Hi @nivesh,
The #config-sync VPN is an internal, automatically created VPN that helps tonfacilitate the config-sync process under the hood. It is not possible to back it up nor is there any real reason to do so.
If you want to backup of all message VPN configuration, they can use the CLI command “show current-config
message-vpn *” as discussed here: Generating CLI Config Sequence Output
Hope that helps!