The vents are how your furnace gets return air. Make sure you keep the basement doors closed. The furnace should heat that area plus the underbelly to keep pipes and tanks from freezing. The vents can be sealed off if you are never in conditions that you need the furnace.
Zack and Donna plus Millie and Ranger
2012 Super Duty
"Life is too short to stay indoors, enjoy the ride!"
I installed 3/4" thick fiberglass insulation (pipe wrap) over the back of the two vents which cut the cold air right down. It still allows air to flow through, no issues with the furnace. These things are not even close to "air tight" so not even sure if it would matter if you sealed them up considering.........