I would agree with the other posts. I don't think VTL can replace tape when it comes to long term retention of data. Not just the power, but also the floor space. Tape is good if you want to take it offsite and let it sit on a shelf for a few years.
Granted with SnapVault (or even VTL) disk based "backups" are a reality today. But disk based "archive" is not. There are some customers that do not have an archive need. In those cases where they maybe keep backups around for 2-6 weeks (or even 2-4 months), then disk is a viable alternative to tape for a number of reasons (reliability, restore speed, scaling, etc).
In general we are seeing most customer environments keeping tape as part of the backup process. Usually there's already an investment in tape technology and the need to be able to retrieve older archived data. VTL can help with the scaling and longevity of tape libaries, so if that's a concern or goal then it may still be a good item to add to your infrastructure. The main driver for most VTLs is primary backup, but given that your primary backup is already met with Snapvault then your current library will keep functioning as is.
Andy