I think the biggest difference other then the protocol is the tiered nature of vitess. Vitess has a different process that serves as the gateway and router while I believe Citis uses a plug-in inside postgres, and uses postgres to handle the connections.

Not knowing more about how Citus does it's sharding indexing I am sure there are differences there but I don't think I could explain them. Vitess has a very flexible and extendable sharding index that gives a lot of control over data locality to the user.