I don't believe there are any frontloaders that never vibrate. Some brands/models claim to have an antivibration feature, which typically works via sensors that determine at what spin RPM vibration is excessive and rev the motor to quickly get past that point. But, every load is different and compensation sensors can work only to a point so some degree of vibration still will occur at some moments.

I'd consider reinforcing the floor under where the units are going to be installed. Advantech is one subfloor product. Glue and screws.

Consumer Reports measures vibration, "usage on a suspended wooden floor", in its washing machine ratings.

http://www.consumerreports.org/join $10/month digital.

Also note that vibration can change over time, as bearings or suspensions fail. We initially had a Maytag stacked washer/dryer in this house. The first years, it worked great, but a little while later it started vibrating ever more and eventually it could never enter the spin cycle without tripping the imbalance sensor.

On the other hand, our current stacked combination is made by Electrolux and, knock on wood, it is about as vibration free now as it was the day we bought it. Overall pretty happy with it. But not sure if it meets the really strict needs of a second-floor installation in a house that already has damaged lath and plaster.

I know that they make heavy rubberized mats that you can put underneath your washer. Those are great for reducing high-frequency vibrations (i.e. noise), but I am not sure how much they would help with low-frequency vibrations (i.e. the whole machine shaking).