Cold plunging, or dunking one’s body into frigid water, is how the 24-year-old stunner likes to get gorgeous before hitting the catwalk for the industry’s biggest fashion houses, including Chanel, Versace and, most recently, Christian Siriano during the brand’s NYFW show at the Empire State Building last week. She told The Post that her regular icy dips — for as much as 10 minutes at a time — are her “ultimate beauty tip,” especially before photoshoots and events. In fact, Vonnahme, who recently began cold-plunging on a lark while spending time in the Hamptons during colder months, claims it’s enough...