Yes, if there's a sufficient build-up/collection of moisture that 'runs', then it will pool and look more like moisture. (IT's a relative 'thing'. I suspect you simply mean condensation is just foggy windows whereas moisture is condensation that's collecting enough that it starts to run (and pool/collect.))
The moisture that's rising up through the ground will condense on the underside of the plastic. THIS is what you want. It's good to hear you don't have water pooling on TOP of the plastic. So you're OK here. And with good sloping as you describe, it sounds like the only 'water' you're getting is from condensation. Again, it sounds like you're OK here ...
As for adding more vents to the crawlspace, you didn't say why you did this. If it's to vent MORE of the moisture out that's condensing on the underside of the plastic, I'm sure you see that the results are the same. Hence, you'd be OK to 'plug up' the vents you've added. You may hear from 'The Insulator' who will recommend you 'box up' the crawl completely. (As an aside, there are differences of opinions as to how to deal with crawls and moisture and so on. Here in the Northease, crawls are vented all the time when they're unheated ...)
As for your heat 'falling through the floor', it's probably more like cool air is being DRAWN up into the heated house, and the heat is getting 'pushed/drawn' up and out. (Hot air doesn't fall - It rises. This is where I'm coming from in my 1st statement in this paragraph.) I wouldn't recommend the foil-backed insulation. That's more of a radiant barrier and used in attics and such. WHen insulating the crawl, go with the Kraft-sided insulation where you install the batt with the Kraft side UP (NOT facing the crawlspace.) You want the vapor barrier (the Kraft) to be installed such that if faces the HEATED part of the house. (Do you understand what I'm saying?) You didn't say how 'high' your crawl is. If it's a few feet, that is good because you want as much room to work as possible.
When you're inside the crawl, where you're looking UP at the floor, push the insulation up with the Kraft side UP. Then, to keep the insulation in place, you have a couple of options. (One option I don't, personally, recommend is those 'wire hangers'.) You can nail/tack/staple up chicken wire, or you can bang in some roofing nails, or something similiar to the BOTTOM part of the joist and string-around some string/wire like a spider web. You may have to do this as you go if the insulation won't stay up for you. You can do it 'loosely' at first, then go back and 'double-up' the process. Just DON'T go pressing/compacting the insulation up there. It needs to be 'luffed' to be effective.
This may not cure all your cold air problems in the floor. Air will still manage to get pulled into the house from there. WHat you may need to do is get on your hands and knees and 'feel around' where the cold air is being drawn up. Then, if possible, either from below or above, 'plug' the source w/some hand-rolled insulation. If you need more info, post up.