What I don't get is, why are they putting vents in and then sealing them?

They have to remove the existing vents to get the rock under the house.I assume that they will be ruined by removal and need replacement.

I've checked in the yellow pages and I can't find a category for companies that do this kind of work. I'd like to get another estimate or some guidance.

If I may ask:

where is the water coming from, up from the ground or the humid air?

what kind of floor does the crawl space have, concrete, dirt,...?

I believe that the ground is the predominant source. We have sandy/clay soil covered by a 4 mil vapor barrier.

If it is ground water the first four items on your list should take care of it. On the last item, the vents, I hesitate because if you run dehumidifiers you should seal the area. If you have vents you should insulate the floor and not run dehumidifiers.

Doesn.t do any good to run air conditioning if the door is wide open and basically thats what a dehumidifier does.

Vented crawlspaces are on leading building scientist Dr. Joe Lstiburek's Top Ten Dumb Things to do in the South

Here is a link that might be useful: Designing Closed Crawlspaces

What if you have a slab and you think there is a creek or pockets of water which shift as the ground shifts?