I'm using CommVault to backup our SQL Servers. Unlike Bill and Jay we've purchased the SQL Agent so that the CommVault system can do the backups directly.
I'm not sure how the Veritas backup works, but the CommVault agent logs into the SQL Server and creates a virtual device on the server, pointing that virtual device, to a tape drive on the tape unit. Then it runs a normal backup database or backup log command using the virtual device it created as the backup device.
Denny