Afaik the vibration when plugging in occurs only if the Silent HW switch from the top left edge is activated (shows orange color). Try to put it on Ring mode and see if it still vibrates when plugging the charger.
Hope this helps.
It's a 5S on iOS 10. Seems that I have to turn off he vibrate in silent mode and the hit the mute switch before plugging it in. I lose the vibration of course when using the mute switch set up like this.