-- If you're going to use a shop-vac, no need to use an extension because
-- there's no need to go into the crawl space. Go where the dryer is
-- hook it up there.
The OP said "The vent is run under the house, about 10 feet to the outside wall."
To which I said: Can you get a 2" shop vac hose down there, maybe with an extension from another shop vac hose?
I'm talking about a extension on the vac hose, not the power cord.
The hose on my shop is barely 10' long with both straight tubes attached to the flexible section. Considering the hose has to be attached to the vac, and considering the straight pieces probably can't be pushed into the vent, I'm guessing he'll need an extension if he wants to blow (or suck) out the entire length of the vent from one location.