Vitess v14 introduced a new, strongly-typed gRPC interface for cluster management, called VtctldServer.
You can refer to the original RFC for details, but the essential difference is the legacy VtctlServer implementation had a single, streaming RPC with the signature:
In order to enable the new service interface, add grpc-vtctld to the list of services in the --service_map flag provided to vtctld.
Both the new and old interfaces may be run from the same vtctld instance, so during transition, most users will set --service_map="grpc-vtctl,grpc-vtctld".
For the full list of commands, as well as the flags they support, you can refer to the client documentation.
Not all commands are currently implemented, but both the old (vtctlclient) and new (vtctldclient) clients provide shim mechanisms to use the new and old interfaces, respectively.
That is to say: vtctlclient VtctldCommand ... allows you to run new vtctldclient CLI commands, and vtctldclient LegacyVtctlCommand ... allows you to run old vtctlclient CLI commands.
For more details, refer to the documentation.