Yes, Backup Exec for Windows Servers does support Virtual Tape Libraries.  This document provides details on the licensing requirements and general usage guidelines. For compatibility information on specific VTL models, please see the Related Documents section of this TechNote below.

Licensing requirements for using Backup Exec with a Virtual Tape Library

A Virtual Tape Library is typically a disk array that mimics the behavior of a tape library. These disk-based systems emulate a library with multiple tape drives. Because Backup Exec (tm) does not know they are not actual tape libraries, in a typical SAN environment where the virtual tape Library is connected via Fibre Channel technology, a Shared Storage Option (SSO) license is functionally required for virtual drive sharing purposes between different media servers.

In addition to the SSO licensing requirement, Backup Exec must also be licensed with a Library Expansion Option (LEO) license for each additional virtual drive, in the same manner an actual tape library would be. Please check the Shared Storage Option (SSO) Hardware Compatibility List (HCL) for Backup Exec for Windows Servers in the Related Documents section of this TechNote.

General Usage Guidelines of Backup Exec for Windows Servers and Virtual Tape Libraries with Drives

The virtual tape device support information is listed below. This information is organized by vendor and provides information on the level of software tested, inquiry string (vendor ID + product ID), and any other important notes about the hardware and its compatibility.
Please Note:
1. The virtual tape device software listed in this section are the only software levels that are supported.
2. Configure the virtual device with the unique inquiry string as listed in this document. Emulating other devices is not supported. The inquiry string should not be that of a real device even if that device is supported by Backup Exec.
3. For each virtual library only the virtual tape drives that are listed are supported. This information can be found with each virtual library in the below section. Other virtual tape drive types are not supported even if the real device it is emulating is supported with Backup Exec.
4. Do not fill up the disk space on a virtual device. Doing so may cause unpredictable behavior of the device. The device vendor will then have to recover from any errors that result from a full disk.
5. When moving virtual volumes to physical media it works best if the virtual device has a volume size that is dynamic. Make sure that the virtual volume is slightly smaller than the physical volume. When you migrate the data to physical media you will need to ensure that there is enough space on the physical device in order to accommodate the virtual volume size.
6. It is highly recommended that Backup Exec move virtual volumes to physical media so that the data can be restored directly from physical media rather than needing to utilize the virtual device as an intermediary restoration point. If the virtual tape library moves virtual volumes to physical media Backup Exec is not aware of the physical media and may not be able to recover that data directly from that physical media.
7. When a virtual volume becomes overwriteable, the virtual device is not aware that the data is no longer needed. When Backup Exec overwrites that virtual volume the virtual library is aware that the rest of the data is not needed and it can free up the disk space. Because of this, the virtual device may appear to be much closer to capacity than it really is.
8. Tested iSCSI versions of virtual tape devices (if any) are listed in the "iSCSI Configurations" section. Please reference the limitations listed in that section.
9. The Drive Manufacturer, Drive Model, and other details for the drive are that of the emulated tape device. The media type in actuality is a disk type.