I have a similar situation, except dryer isn't stacked, and the machines are in a hallway, not a closet. My dryer duct goes through the floor into the basement, and runs along the basement ceiling to the closest exterior wall. The very top of my basement is above grade, so it was possible to run the duct right out through the wall, terminated by a louvered vent cap. It comes out very close to the ground, though, which isn't ideal and might be against code. Recently I had one of these installed in place of the louvered vent cap:

http://www.amazon.com/gp/product/B00009W3I4/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

which brings it higher above the ground. It isn't available now, which is unfortunate because it's a very good product. There may be similar items still on the market.

I am not an expert, maybe someone else will post that knows more than me. Here is my experience with my set that I put into my bathroom during a remodel:

Check your manual or go online to find it. There should be vent length recommendations in the manual. There is also a standard calculation to determine the length of vent pipe you can use. There are several considerations. One, for example, requires that each 90 degree turn = 5 feet of pipe. My max dryer vent pipe was listed as 33' but the actual length is no where near that long. My three 90 degree turns used 15' of vent in the calculation from the start. I have about 13' of actual length to get to the outside wall, so that adds up to 28' run of vent. If you can't make the distances work you may have to get a buster fan that is installed in the pipe to help.

Another concern with putting the W/D upstairs is the flooring. Is this stacked set going along a load bearing wall? My set requires this setup. If you put it in the middle of a joist span it will bounce too much. Front loaders create a lot of energy with the high rotations.

Opps, I see that suburbanmd beat my post. Like their vent mine also went down into the basement and along the floor joist and out the side of the house. Mine is plenty high above grade in my location.

Thanks Enduring. The left hand wall of the closet is right above a load bearing beam. Should be solid enough don't you think?

The best exhaust path for a dryer in *any* install situation is as short as possible with minimal number of turns.

There's really no way to predict what will happen regards to vibration until the machines are in place and running. My neighbor remodeled a 100-years-old house for rental. The washer & dryer are in a hall closet. First floor. Old-style toploader. Set square in the space, perfectly level and stable, it vibrates and walks itself during spin until it "finds" and settles at a "happy" angle and/or rests against a corner of the dryer. The finish on the side of the dryer that abuts to the washer is worn from the contact vibration.