is the vibration cyclical? is there a pattern to it? if there is then its one of your plugs, the plug is dirty, not gapped right, ect. on a 4 cylinder if i know the firing order i can almost tell you which plugs to check just by feeling and listening to it :-D im not joking! i knew that i had to regap #1 or #4 in my car when i had a vibration at idle and i knew that my GF's escort had 2 dirty, improperly gapped, or incorrect plugs. and sure enough 2 of the plugs were 2 heat ranges too cold.
i'll try to explain how to feel for this the best i can. put your hand on the engine when its at idle in D with the wheels chocked and parking brake on or someone in the car.
with your hand on the engine feel if theres a pattern it may be like ...-. ...-. ...-. which would indicate that its one of the 4 plugs that has an issue. if it feels more like ..-- or .-.- then it would be 2 plugs. it takes a while to be able to sense these things and you need a keen knowledge of how the engine operates. ive been around small engines and cars for a little while but i pick up and learn things like this really fast.
you can also listen for slight misfire at the tail pipe, there will be a different tone when it misses.