Keyboard keeps vibrating, even though "vibrate on tap" is off?
It depends on your version of Android. 

Samsung Touchwiz-based phones and many others have the ability to control key press vibration intensity. Some more pure ASOP phone do not, such as official Google phones, or LineageOS. 

One answer is to change keyboard. Both SwiftKey and GBoard offer the option to override the system sound and vibration settings.