Viber is an incredible app and keeps check on every activity. If another person takes a screenshot of the conversation than Viber will instantly notify you about this activity.
For this, you have to check to mark the secret messages. Here messages remain for some time till you delete them. If anyone screenshots your conversation, Viber will immediately notify you and keep you alert about this activity.
So if someone wants to capture your conversation you can immediately get the notification and warn him for doing so.