Maybe a temporary software-related issue prevents your device from vibrating. Try restarting your device and your device should start vibrating.
See the don't know how to restart my device guide if you are not sure how to restart your device.
If your device doesn't ring when you receive notifications from an app, it is possible that its notification sounds and the vibrations are disabled. In order to enable them, you should go in the app's notification settings.
If your Wi-Fi or Mobile data is disabled, you won't be able to receive calls through apps such as Skype and Viber and your device won't vibrate nor ring.
Check out the don't know how to enable/disable Wi-Fi or don't know how to enable/disable Mobile/Cellular data guides in order to learn how.
Maybe your iOS device doesn't vibrate when you receive calls from an app (e.g. Viber or Skype) because the notifications for that app are disabled in the iPhone's Settings. In order to enable the notifications, follow the steps below:
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.