For the vents you want to close up, just cut a piece of 1/2" or 3/4" foam board and trim to fit. Make sure you leave one vent at the top cracked and one lower opening. If you feel a draft from them, you probably have them open too much.
If you don't have thermal windows, add the foam board to the windows you don't care about seeing through. For the windows you do want to see through, cut a piece of acrylic panel, add a gasket and then tape them in place. Will almost completely eliminate the draft you feel by sitting by the window.
If I have power, I also have a small fan force 600 watt heater I use. Helps keep the cold at bay when it's on.