The eccentric British designer was speaking to reporters before her show at London Fashion Week, where she handed out a "climate revolution charter" which called on people to "buy less, choose well, make it last".
Westwood, a long-time environmental activist, was asked what advice she may have for the Duchess of Cambridge, who has become a style icon since marrying into the British royal family in 2011.
"I don't have any advice to her, except I think it would be great if she wore the same clothes over and again, because that's very good for the environment and it would send out a very nice message," Westwood said.
Wearing a t-shirt with an image of herself looking like WikiLeaks founder Julian Assange, the 71-year-old added: "If you're going to all that trouble to get an outfit that suits you, then you should keep on wearing it.
"I mean, you don't have to have a red outfit one day and then something almost the same in blue the next."
Westwood said she kept the latest collection for her diffusion line, Vivienne Westwood Red Label, deliberately short but there was enough to show her usual craftsmanship and bold use of colours mixed with a good dose of humour.
The clothes were elegant and wearable, with a few out-there items such as silver shimmering harem pants matched with a tight, iridescent jacket.
In contrast with the blank walls of the Saatchi Gallery, the make-up was wild -- the models' faces were painted with exaggerated eyebrows, eyes and lips annotated with splotches of turquoise, red and yellow.
The third day of London Fashion Week also saw the autumn/winter collection by one of Kate's favourite designers, Alice Temperley.
Temperley London showed a soft but steely range of pristine day outfits and sweeping floor-length ballgowns based on the style of Tippi Hedren, the ice cool star of Hitchcock movies "The Birds" and "Marnie".
Temperley said this season's clothes were aimed at "an alpha female with a darker side, but still luxuriously chic and ultra feminine".
Figure-hugging cashmere tops were matched with full skirts, silk scarves with sheer, embroidered blouses, checked wool coats and engineered prints with flashes of gold and emerald among a classic palette of cream, navy and cobalt blue.
Impeccable pulled-back hair, statement sunglasses, towering stiletto heels and stockings with a perfect seam completed the image of a leading lady who gives away just enough to keep her fans guessing, but nothing more.
Topshop Unique, the only high street brand to show at Fashion Week, presented oversized shapes, utilising contrasting materials in the concrete setting of the old tanks at the Tate Modern.
The collection featured large coats, tight skirts which reached mid-calf boots with thick heels and fluffy textures sitting alongside rigid plastic materials.