If you put down more plastic, I suggest Tu-Tuff. Find it at www.efi.org. It is a far superior product that is much stronger and doesn't have the pin-holes common to hardware store poly. Overlap a nd tape the seams and around any penetrations. Run it up the sides of the foundation and tape.
I agree with blocking the vents. If you are running at 50 oF with vents, you're expending a lot of energy to keep the area warm. With the vents blocked, you can probably afford to add duct insulation to cut down the heat loss.