Vibration as in, you feel the car\'s body vibrate? Vibration that spreads to the cabin of the car?
Since it sounds like the vibration is present only when the car is in gear at at a stand-still (idling with the brake applied) - could be a number of things. Assuming that the engine is perfectly tuned up - any vibrations that you feel are probably from worn engine mounts. Engine mounts are more likely, if the engine \"rock\" back and forth while you powerbrake the drivetrain. Powerbraking is basically placing the car in gear, standing on the brake pedal (e-brake engaged) and revving the engine quickly a few times. Don\'t need to run it to redline, just a quick couple of presses on the accelerator pedal is enough. If the engine moves several inches back and forth when you \"powerbrake\" the drivetrain - there is a good chance the mounts are worn out. They would have to be replaced.
If the tune of the engine is not known or if you haven;t had a recent tuneup - could be anything from clogged or dirty injectors, dirty or gummed up throttle body, clogged air filter, dirty MAF and IAT sensor, and/or worn sparkplugs. The OEM sparkplugs are long life iridium plugs - they are designed to last up to 120K miles before requiring replacement, but some cars will go through them a little quicker. Could be the plugs just need a good cleaning (a quick run down a highway) to blow off the accumulated deposits. If the deposits are excessive or if the plugs are heavily worn - you will need to replace them. A bottle of good injector cleaner (Chevron, Redline SI-1, Lubromoly Valve Clean, etc.) - can help clear mildly dirty injectors and may help clean up some combustion chamber / intake valves from carbon deposits. The MAF/IAT sensor is located in the airbox - generally not a serviceable item - many have greatly improved engine performance by cleaning this sensor. There are many DIY articles online. Cleaning the throttlebody will have a significant impact on idle quality - they make several kits that you can purchase to clean it yourself. Or grab a spray bottle of throttlebody cleaner (non-clorinated and O2 sensor safe), an old rag or toothbrush, and a little elbow grease will get the deposits off. Again - many articles can be found only to give you guidence - if needed.