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I needed to export a few hundred users from Google Vault, and as far as I know there is no public API to do this function!
I created the following BASH script which can do this work.
Google Vault seems to limit you to 15 running export functions at once, so you still have to chunk this up.
First using something like Fiddler or Burp Suite, capture the cookie for your Google session, and the GUID for the matter you want to export the data from.
Then create vault_users.csv which contains each email address you want to export on a single line:
Place this in the same folder as runme.sh
Check out the code here: https://github.com/planetcr/vaultbash/