mine idles at like 400 rpm in the summer and it vibrates my whole car only cuz i messed with my fuel pressure regulator. that vibration is cuz its getting very little fuel to the car so its like almost dying but i would think the vibration would be at like 500 or less, but maybe its different in other cars. most likely its perfectly fine, if it is something its probably good that your car idles that low, saving gas... etc..