iPhone vibrates on every Viber text message received
It looks like the feedback has been acted upon, and it is now possible to turn off vibrations and sounds while you are using the app already. A very welcome change indeed! (Not sure which version it was implemented in) You can do it by going to "More" tab on Viber, and opening Settings -> Notifications, and turn off "In-App vibrations". See the official Viber documentation here. Excerpt:


  In-App Vibrate
  
  When this option is enabled, you will receive a vibration for every incoming notification.