im wondering why when my car sits at idle, without the headlights on, it doesn't have much vibration, but when i turn the headlights on, it starts vibrating very hard????

 

That's because the headlights draw current which the alternator has to replace. The alternator loads down the engine. There is an 'Electric Load Detector' (ELD) that should step up the idle to prevent this. I would check out the ELD, and maybe the battery, too, while you're at it.