I Live in SE PA. Our summers aren't that different from yours. We don't install fans no matter what. Let's assume you have 1 vent and put a fan in it. What is the SOURCE of the air that you 'push out'? Will it come from the main house? Will it come from the little cracks between the sill and the ribbon board? You have to realize that air OUT must be replaced with air IN. Otherwise, you create NEGATIVE pressure in the crawlspace and burn up your fan.
Now, let's assume you have 2 vents and you install a fan in just one vent. The air OUT will be replaced by air that's drawn IN from the 2nd vent. This air is humid and will cool and condense on whatever it passes as it replaces the air that's pushed out. This will exacerbate your problem. Before you do any work, spend $10 and buy yourself one of those thermometers from Radio Shack that has a thermo 'extension' on it. Unravel the wire and put it in the crawlspace. Place the readout in your house (if it reaches. Otherwise, unhook it and only hook it up when you want to get a reading. Try and place it in the 'middle' of the crawlspace. (Maybe there's a duct or a crack or a hole of some sort in the main house that you can 'slip' in the sensor. See how close the temp in the crawl is to the air outside. I bet it isn't any higher than 60 degrees or it isn't a crawlspace!
I know you will do what you want but believe me, you're approaching this the wrong way. In short, the crawl is cooler than the outside and ANY moisture, drawn or natural, will condense in the cooler space (the crawlspace.) Think about it. Why is your crawl damp? If it't not posing a problem, leave well enough alone. If it IS causing the problem, don't bandaid it; fix the CAUSE of the moisture. (Vapor barrier, repair your gutters, improve drainage around the crawl, etc., etc..) Your house will 'feel' much better. Run this entire Post and all its follow-ups by your contractor. See what 'holes' they can shoot in it. Then, get back to me/us and we'll go from there.
PS: I'm out for now and will return on Monday morning ...