Chris,
This may be a permissions problem on the usage.asp. I usually recommend that you disable integrated windows permissions on this asp file (forcing you to logon with a \) account as a troubleshooting step. It's pretty easy to do/undo. Just go in to IIS, find the usage.asp in the EnterpriseVault web directory, right click for props, select the security tab, client security, and uncheck integrated windows auth (leaving basic on).
relaunch the page, and be sure to logon with the domain\VSA account. See if you get your proper values then,
micah