Yes, Google Vault supports classic Hangouts messages that are retained if chat history is turned on.
- History-on (archived) classic Hangouts messages are retained according to the chat holds and retention rules for classic Hangouts in Google Vault. Archived messages can be searched, exported, and placed on hold for eDiscovery purposes.
- Shortly after a retention period expires, archived messages are expunged from classic Hangouts, and the user's Gmail mailbox. Chat messages will no longer be searchable or exportable by Vault users with the appropriate privileges.
Yes. Learn how to install classic Hangouts on all your devices.
If you want to use the photo-sharing functionality in classic Hangouts, you need to turn on the Google Photos service in your Admin console.
Note: Google Photos is an additional service that is not subject to the terms of your G Suite account agreement.