About default Enterprise Vault backup options
You can select a default backup method that is based on the type of Enterprise Vault database that you want to back up.
For Directory and Monitoring databases, and the Enterprise Vault 8.x Audit and FSA Reporting databases, the Full backup method is substituted for the differential backup method.
When you use the incremental backup method for the Enterprise Vault databases, the transaction logs are backed up and then truncated.
See About backup methods for Enterprise Vault backup jobs.