I had this question too, even their "Database Scaling" course doesn't get to it.

https://vitess.io/docs/faq/sharding/advanced/

Looks like it's not drop-in, you need to heavily modify your data and indexes structurally to make sharding performant. Cross-shard joins are possible but need to be designed for explicitly (to be fair, indexes also need to be designed explicitly in regular SQL). IDK if a drop-in solution is possible or if this is an information-theoretical limitation.