Firstly go into Messenger, and then
Chats
On the chat, touch the bottom of the chat and
swipe up to turn on vanish mode
Note: If your phone has a swipe up feature to show settings, like with iOS devices, swipe from within the messenger window, and not the absolute bottom of your screen.
Turn Off Vanish Mode
To turn vanish mode off, repeat the same actions as above, or tap the
Turn Off Vanish Mode button
How to See an Already Vanished Message?
Once the message has disappeared you cannot recover it, nor are you able to copy, save or forward a vanishing message before it has disappeared. The only way to save a message is with a screenshot.