My answer to the question whether one should change a classic tape library into a virtual tape library is nearly the same as for the question regarding the use of VTLs instead of backup to disk or deduplication storages. Because we have the same issue here: VTLs are, oversimplified, just file servers. So they have the same benefits as classical backup to disk devices have over physical tape libraries. But they also have the same disadvantages: you cannot remove the media to store is offsite.
Of course there are arguments, that the VTL could be placed into a different fire protection area than the backup server. But that’s nothing you can’t do with a physical tape library.
Therefore I’m asking the same question as I do in my post VTL vs. Disk: What reason do you have, to buy a VTL? If you don’t want or need to bring your backup media offsite, use a simple backup to disk device as backup target. And if you want or need to bring your backup media offsite, a VTL isn’t an alternative anyway.
Besides this you should bear in mind, that many VTLs have their own media management that is not necessarily synchronized with Backup Exec. If this is the case, it’s just a matter of time, until the inventory data within Backup Exec starts to differ from the one in the VTL. And then it’s up to you, the administrator, to decide, in which system you want to trust.