How can I  vent this dryer?
If you have it too long with too many bends you will be creating a fire hazzard.  You need to be able to clean out that dryer vent.  If you have it running 15' one way and 10' another with 90 degree turns, then you might run into fire safty issues.  If you can put it up against an outside wall, make your hole there on the second story.  This will save you money, reduce fire risk greatly, and you wont have to pay a small fortune to have someone run a 30' snake to clean it every year.