Others may differ but I don't suggest you put fans in the vents. How did you come to this 'end'???
In doing this, you're PULLING warm, moist OUTSIDE air into a cool crawlspace. And do you know what happens when warm, moist air comes in contact w/a cool surface??? The moisture in the warm air CONDENSES out and settles on the cool surface. Hence, moisture problems, along with wet and dry rot and insect problems and so on. With the vents, which are usually installed in SE PA in a ratio of 1 per 250 sq. ft. of space, are left to Mother Nature to 'control' air flow.
As far as a vapor barrier, yes, lay down some 6 mil thick Poly on top of the 'ground'. Then, lay something like bricks or gravel or pressure-treated 2x4s along the entire perimeter to keep the plastic down. It's OK for any moisture that rises up through the ground to condense out on the BOTTOM side of the plastic. As for water that ends up on TOP of the plastic (from whatever source), let Mother Nature dry it out. That's what the vents are for. And it can be a slooooooow process. But don't worry - That's not a 'bad thing'.
And about your insulation under there, be sure to use a Kraft-sided insulation that's installed with the Kraft-side 'facing' the heated room (above.) The Kraft is NOT to face towards the ground!
Yes, fix your gutters, improve your sloping, divert the downspout water WAY AWAY from the foundation, and so on all around the house. All that's been described here should dramatically reduce, or even eliminate, your problem. My best to ya and hope this helps.
Jay J -Moderator
PS: If Steve: sees your post, he may follow-up on it too. He's from Texas (which is why I put 'Texas' in my Subject Line ...)