Vault uses merkle sync when two clusters get too out of sync for typical replication. Might not be 1:1, but might explain why it'd be useful for a relay. Here is a snippet from HashiCorp's documentation around what this enables:

"Any time a key is updated or deleted, the merkle index is updated to reflect the change. When a secondary needs to reconcile with a primary, they compare their merkle indexes to determine which keys are out of sync. The structure of the index allows this to be done very efficiently, usually requiring only two round trips and a small amount of data. The secondary uses this information to reconcile and then switches back into WAL streaming mode."

https://developer.hashicorp.com/vault/docs/internals/replication