If you are running Vault in a 32-bit environment and trying to view PDF's you may see this message appear in the Preview pane.
"Unable to view the file selected. Click the 'Open' button to view with an external viewer."
The Vault preview mechanism is the same as used by Outlook 2007 and 2010. Both use a PDF preview handler which is controlled by Adobe themselves. A discussion on this topic can be found here.
Until Adobe fix this issue in 32 bit windows, the only workarounds are to either use a 64 bit OS or open the file in Adobe Acrobat Reader itself.