I suppose the floor area under the house is bare soil, i.e., the crawl
space. I think, in the cases of no vents, a moisture barrier (lots of
visquene) is placed on the ground, to keep the moisture/excess high
humidity from rising from the soil..... Making sure the soil is
sealed off well.
You might not want to permanently seal the vents, but just close them
off (after the visquene treatment). If you would ever need to
ventilate the area, they would still be at your service.
I'm several states away from you and your scenario is not applicable,
here, but my sister is near Statesville, NC. Her new home has no
vents and has visquene laid in the crawl space. You might want to
speak to a contractor to make sure my reasoning is correct or not.
Most of her crawl space is about 4'-5' high, so one can walk around,
in there. Her water heater is there, as is very easy access to
heating/cooling duct work, plumbing, dryer duct (to outer wall. The
dryer moisture is not vented into the crawl space), etc. I'm no
contractor, but when I visit her, I give those utilities an
inspection, for her. The visquene makes for a relatively clean work
area, under there, also, rather than working on bare soil.