I'm kinda agreeing w/Steve:, sight unseen. I think the dampness is coming from the ground. If the ground isn't currently covered w/6 mil plastic, then it should be. Any edge, both the borders and the edges that you'll wrap around the piers, should be 'held down' with gravel. You can buy the plastic in the Paint Aisle at the Home Center (6 mil thick, clear or black.)
As far as venting the space, you may not have to if installing the plastic sufficiently prevents moisture from rising. What little comes up, may be enough to 'tolerate'.
RE: The plywood subfloor - I, too, think it was just to keep the 'wet concrete' from going all over the place. I don't think it's structural either. I'm going to be the concrete is reinforced w/steel.
RE: Other sources of moisture - If the porch is warm and the concrete is cool/cold, then the moisture in the warm air will condense-out onto the floor. When warm, moist air comes in contact w/a cold surface, the moisture condenses out. You see this all the time in Winter when you breath outside in the cold, or breath on a cold car window, or something.
I'd hold off on venting for now until you cover the crawl's floor. Then look into the 'conditions' in the porch to see if it is contributing to the moisture on the floor. As a test, you could cut a piece of 2' x 2' plastic (of any type w/no holes), and duct-tape it to the concrete COMPLETELY on all 4 sides, and wait a couple of days to see if moisture is condensing out on the UNDERSIDE of the plastic. If it is, then the source of the moisture is from below. If the moisture is condensing out on the TOP of the plastic, the source is the warm moist air in the porch. Oh, the effect you're experiencing is VERY common on south-facing porches. They get REALLY warm during the day and cool at night, causing moisture to condense out. The fix, here, is different.