Since all the Vaultastic plans allow unlimited storage, this is not too important. However, you can get email reports on the storage usage of your domain.
Vaultastic provides various reports Viz. Usage Summary, Active Users, Automatic User Creation, Storage Usage, and Missing Users Report which you can subscribe to receive in your mailbox either Daily, Weekly, or Monthly.
The Storage Usage report gives you the total storage used by all the accounts on your Vaultastic domain.