Cornelia Guest, the daughter of socialite and style icon C.Z. Guest who was herself memorably dubbed "Deb of the Decade" in the '80s, is set to put her vegan principles into practice with a forthcoming bag and accessories range. The Donna Karan brand ambassador -- who every major American designer has dressed since she ran with the Warhol crowd in her youth -- also runs an events business, which will be catering meat-and-dairy-free meals (including her signature cookies) for Thanksgiving.

StyleList spoke with Guest as she prepared for what could be her last Thanksgiving at Templeton, her family's gracious estate in Old Westbury, N.Y., which is on the market.

"Thanksgiving for a vegan is a really easy holiday, because you have so many wonderful root vegetables that are served," she said. "Aside from the turkey and the ham, so much of the food you would normally serve is vegan."

Guest said she uses olive oil rather than butter for classic dishes like mashed potatoes, and sources vegan marshmallows for her sweet potato pie. "This is our time of harvest, and everything you can harvest is vegan, because it comes from Mother Nature," she said.

Guest said she became vegan after becoming more aware of health when her mother died of ovarian cancer in 2003. That's something she has in common with Karan, who became a health activist after losing her husband to lung cancer in 2001.

"I'm her brand ambassador; I personally don't wear leather, but her clothes are beautiful and she's incredibly supportive of women and what women are doing," says Guest, who has held the role for about a year.

Guest is in the planning stage for her "vegan and organic" accessories range, and says it's too soon to announce details of the designer with whom she's collaborating. But her life at the top of New York society has meant that she has known many of the top names in international fashion.

"Halston was unbelievable," she remembers. "And he would just drape, and sewed a dress on me once. I'll never forget, it was amazing. Valentino sent me the most beautiful one-shoulder dress for his 45th anniversary at the Temple of Venus [in Rome, in 2007], and getting fitted for that was wonderful."

"And yet, I'm at the Gap all the time because you can't get a better pair of leggings or blue jeans, and I love Gap sweaters. I love wearing these beautiful clothes and I'm comfortable in them in the situation, but I'm a country girl and I'm happiest in jeans with a T-shirt on in my garden."

So, quite a change from her "Deb of the Decade" years?

"Things have changed a lot," she said with a laugh. "And we all grow up don't we? We hope!"