The only thing that would make sense to me would be if you use power saver mode sometimes. It's a known cause from previous issues exactly like this, where the problem seems intermittent. This is because the user would manually use power saver (or it would kick in when the battery was low) which switched off vibration feedback - which is normal.. however what wasn't expected was that when power saver was turned off (whether automatically shutting off after charging or turned off manually) it did not restore vibration feedback to the "on" setting. Thus they would have to turn vibration feedback on again in the settings. If this isn't the case with you, I'd say your situation is fairly unique and booting into recovery, wiping cache to see if the problem persists would be step 1. If that fails then it might be factory reset time.