Hi Evan,
in truth, Backup Vault (BV) is not the legacy solution but it is complementary to Recovery Services Vault (RSV).
Some of the key differences are: BV supports some datasources that RSV does not support (Azure Blobs, Azure Disks, Azure Database for PostgreSQL servers); BV does not transfer data to vault storage for long term retention, it only works at snapshot level (you are only charged for the delta storage costs, not for backup storage).
You should use either BV or RSV depending on your datasources and in some cases you will use both.
Hope that helped.
BR,
Alessio
