Is the Vault in Read only mode?
Are there items in MSMQ, especially the A1 queue?
Run the following command on the Vault Store partition through the command line and look for a process ramping up in Task Manager called StorageFileWatch.
attrib -a *.* /s
This will simulate a backup of the data and you should notice in the Vault Store Usage Page that the item count should start to drop for items awaiting backup.