Why the tablet vibrates even though I have turned off this feature?

Android distinguishes between vibration feedback when scrolling in the menu and if typed on the keyboard. If you want to completely dispense with vibration feedback, so you should make the following settings:

Settings -> (Device) Sounds -> (System) Vibrate on touch

Settings -> (Device) Language & input -> (Keyboard & input methods) Google keyboard settings -> Vibrate on key-press.
(Should you use an alternative keyboard, so they choose instead of Android keyboard this.)