--Diana Vreeland, via Stephen Jamail, Immoderate Style
The above quip is nowhere to be found in Diana Vreeland’s memoir D.V. Instead, it is but one of many off-the-cuff remarks Vreeland made in the company of Stephen Jamail, her devoted right hand man at the Costume Institute. Jamail recalled that Vreeland's nonstop aphorisms, spoken in a voice that “was a cross between a New York cabby and an English duchess,” often left him “scrambling for a scrap of paper to” in a desperate attempt to record them for posterity.
In this case, the quip might not have been exactly lost, just more difficult to uncover. It is also buried in a 2009 issue of Vogue Australia. The periodical’s former editor Sheila Scotter disclosed that she was present when the comment was made and revealed the context. It was Vreeland’s demurral "when asked what was the worst fashion show she had ever attended.”
Back in 1982, when Vreeland was very much alive, Stephen Jamail cooperated with a New York Magazine cover story on his Costume Institute boss, which is available in its entirety via Google Books.