I have a 900 sq.st. home in the cold midwest. All the duct work and piping are in the crawl.
Their is only 1 vent.
This the outside air vent in the foundation of the house. Their is only 1 vent. like this.
By vent do you mean a supply air vent or an outside air vent. If an outside air vent then leave it opened. If a supply air vent well it depends on if your water lines are insulated well enough. In any case I would leave it open too. This will also help take the chill off your floors. It won't hurt a thing.
The vent should NOT be closed for the winter. Depending on data you did not provide, it is possible, but unlikely, that it should be eliminated entirely, but It is either needed year round, or not needed at all.