As said above, the vents are absolutely necessary for proper operation of the refrigerator. You can stuff insulation all around the fridge, except between the back of the unit and the outside wall, air must flow across the back of the unit to cool.
The windows may not be leaking air at all, if it is cold enough out side, the transfer of cold from the outside to the inside, especially if you have single pane windows, can create a draft from the cold air forming on the inside falling to the floor. A good heavy curtain or insulating blinds will help dramatical. As far as putting plastic over the windows, you don't attach it to the window glass, you attach it to the window frame creating an air gap to form an insulating barrier like having double pane windows. Bigger the air gap, the better it works.