Westwood, famed for her use of tartan and Harris Tweed, addressed the debate at last night's Scottish Fashion Awards.
The famed designer was inducted into the awards' Hall Of Fame for her contribution to the industry.
Accepting the honour, Westwood, 73, said an independent Scotland could be "a model to us all ... if it could become a people-friendly society".
Yes Scotland chief executive Blair Jenkins said today: "It's become ever clearer during the past couple of months that Yes Scotland is the campaign with positivity, optimism and a vision for Scotland's future - and with Dame Vivienne's declaration, it turns out that we are also the most fashionable!"
The Ninth annual awards were hosted in central London by founder Tessa Hartmann and Scottish Secretary Alistair Carmichael.
Ms Hartmann said: "We were hugely honoured to induct Dame Vivienne Westwood into the Hall of Fame.
"We are proud that our awards are apolitical and we support freedom of speech. The fashion industry is an expression of creativity, eccentricity and bold statements.
"As such, we are offering a kaleidoscopic canvas on which our nominees and winners can showcase their talents."
Ms Hartmann said it was "no secret" that she personally believes that Scots flourish as part of the union.
"I would be heartbroken to be excluded from a 'British' fashion industry that is envied and revered around the world," she said.
"The Scots have played a vital role in the evolution of British fashion but we would fall apart at the seams without that support, without the incredible commercial opportunities that London offers to up-and-coming designers and without being part of that wonderful heritage."