I'm not sure if I'm following the posts right and don't think I would recommend putting one in a dryer vent, but is anyone referring to a damper of the type that is put in your hvac ducting?
The flapper type vent on the outside should keep out cold air. They are most often used with bathroom vents that go straight out the wall of a house. If the wind blows directly into it, them put a shroud over the flapper vent as is sold in most stores. The dryer air blows out the bottom of the shroud and I haven't heard that it is a big problem as about every house in the country has one of one type or another. Last but not least, dry more clothes.