Condensation still occurs in a vented crawlspace in the winter, it’s just not on the ducts. Typically on the foundation brick walls and band/rim joist.
The temp drops faster than the drying capacity/Air exchange rate of the cold dry air through the vents.
If you want to seal automatic vents it’s a bit more involved that standard mortared in vents. Cut 7x14 pieces and glue/tape these to the back of the vents. This can be done from the outside. Then crawl under and use expanding foam around the vent and cover.
Also serves to hold the vent in place better than spring clips and the factory screws.
If sealing then some type of means to dry and keep the crawlspace dry is necessary in conjunction with a minimum 100% vapor barrier.
