Have you tried to delete the vault from the Terminal using the "sudo" prefix?
Be very careful when using "sudo" command - check everything you enter carefully before hitting return
What else is on the drive? How is the drive formatted?
If the vault is the only thing on the drive I would recommend an erase and format. The drive does not sound like it was set up right or else it was damaged.
If you can't erase/format because of the data on the drive then I would do a disk repair from Disk Utility even if you manage to get rid of the vault.