The plastic vapor barrier I was talking about on the underside of the floor framing was on the warm side of the insulation, given that rigid sheets would then be put on to the bottom afterwards. I figured this would be easier than putting plastic up in to each bay and around the joists before putting fiberglass batts in there. The way I described kept the barrier on the warm side of the insulation, since the entire joist bays would be on the warm side. It just seemed easier to retrofit this way.
As I noted in my original post, a lot depends on other conditions, which you did not describe; your climate, how much moisture gets in and how it gets there. These factors need to be debated when considering whether to seal the vents and use a dehumidifier or to leave the vents alone. Where I live, they make you put vents in but everyone seals them up after final inspection and runs dehumidifiers because we typically don't have a lot of ground moisture getting in there, mostly condensation, and the dehumidifiers work well enough to keep everything nice and dry. Your situation might be different, depending on local conditions and the particulars of your house and site.
I would ALWAYS insulate your (joist) floor. Roughly 15 % of your heat in the winter will be lost to the floor, AFTER insulating. You are probably losing much more than that now. Cooling is even worse. Besides your feet would be too cold in the winter. Unless you live someplace that is always between 60 and 80 degrees all year, I would insulate the floor.