Based on the images, you might get away with using 4 or so screws around the original hole going into the plastic end of the post from the carpeted plank bit. This all depends on if there is much for the threads to bite into behind the plastic.

Look into MVHR as an option to ventilate the property. Will keep it well aired, prevent humidity build-up, and keep the heat in. If you seal up the existing vents without an alternate solution though you will have humidity issues as spray foam insulation is very good at sealing a property.