That is my understanding also. In fact I'm sure that's right. I'm just
uncertain as to whether or not file backups are also created from snapshots
taken by volsnap.sys. They might be.
One way to check is to try it in a dual boot XP/Vista virtual machine using
VPC and just see what happens to backup files after booting into XP. This
would require three virtual hard drives (one for the backup files).