My Iphone 5 has Vibration issue. When I purchased vibration mode was not working, though I didn't notice it while buying. After using for a week suddenly vibration started working and it worked smoothly for couple of weeks. Now suddently it stopped again. Never ever my phone fell down and never ever it was handled improperly, but still dont know what is happening with the Vibrations.
My brother purchased this phone from Austraila but I'm using it in Qatar, so not sure about warantty also.