To Block... or Not to Block Crawlspace Vents during the Winter?

Option One and Two, Depending on Type of Crawl Space:

In simplest terms, there are crawlspaces with water pipes and those without. Crawl spaces with pipes will either have the concrete walls around the perimeter coated with insulation, or the pipes themselves will be insulated or wrapped with heat tape (sometimes both). If your crawlspace has no pipes, or the pipes are self insulated, you can get away with adding insulation to the sub floor to inhibit the cold air from reaching your living space above. If your crawl space has uninsulated pipes and insulation around its walls, it was most likely designed to allow a portion of the heat from the living space above to escape below to keep the pipes from freezing. In this case it would be highly unadvised to insulate your sub floor, as that would be taking away from the one source of heat your crawlspace has. What would be most beneficial for this scenario is beefing up your existing insulation around the concrete walls to keep the area warmer.