Unity picks disk automatically. There are some discussion on this topic already happened.

Example : Unity Drive Selection

Vault drives will be picked last/when no other eligible drives found.

Yes, you can select vault disks and add them to pool.

Those drives are now only the OS/services drives and can be added to the pool. The vault is now an internal M2 SSD in each SP.

so would the svc_reinit remove all the data on the vault drives if they are being used as storage in addition  to flare OS?

Hello sm3013,

Reinitializing the Dell EMC Unity OE should be considered a last resort, and by nature is fully destructive, completely removing all data and configuration that may already exist.