The recommended way to enable Vault Cache searching is using the WDS Search Auto-enable setting in the Exchange Desktop Policy. Alternatively, you can enable the plug-in during installation by including the command line parameter ACTIVATE_WDS_PLUGIN=1. Note that this command line switch is case-sensitive.

For example, the command line for a silent install would be the following:

msiexec /I path_to_installer ACTIVATE_WDS_PLUGIN=1 /qn

where path_to_installer is the path to the required language version of the Enterprise Vault Outlook Add-In and HTTP-only Outlook Add-In MSI file.