Here's another possibility.

fluffy
...
2. Light and airy; soft
http://www.answers.com/fluffy

http://www.thefreedictionary.com/evanescent

Air
This evanescent dress lined with silk organza explores the notion of transparency, light and shade, thanks to the juxtaposition of an opaque black corset and a wide white skirt. Scintillating threads inserted into the fabric of the skirt accentuate reflections and create celestial, airy effects.
http://www.egodesign.ca/en/actualite_details.php?actualite_i...

the world of fashion uses this term to indicate the way the fabric "moves" down the runway (when worn), it "flows"

google "flowing fabric" (1,800.000 results), flowy dress

I'd translate

"they are light, free-flowing, yet utterly/perfectly tailored"

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Nov 23, 2008 ... Paris Hilton looked classy in an eye-catching dress as she posed at an LA bar opening in the wake of her split with rocker Benji Madden.
www.dailymail.co.uk/.../Benji-Newly-single-Paris-Hilton-hit...

NICOLE RICHIE FASHION: Nicole wearing a flowing dressAug 14, 2007 ... Nicole is wearing Christian Louboutin wooden wedges, a grey flowing dress, Mason by Michelle Mason, Chanel sunnies and her Givenchy ...
nicolerichiefashion.blogspot.com/.../nicole-wearing-flowing-dress.html - Cached
Flowing Dress - The Hollywood GossipMar 8, 2010 ... Jennifer Hudson shows off her dress at the 2010 Vanity Fair party. It's held every year after the Academy Awards.
www.thehollywoodgossip.com/gallery/flowing-dress/ - Cached
Flowy Beach Wedding Dress - LoveToKnow WeddingsHeat/Temperature: If your wedding will take place in an area with warm temperatures, a slightly looser or flowing dress will make the heat much more ...
weddings.lovetoknow.com › ... › Weddings › Wedding Dresses - Cached - Similar

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Note added at 3 hrs (2010-11-15 20:45:22 GMT)
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Ghost - Fashion Designer Encyclopedia - clothing, women, shoes ...Orlean, Susan, "The Talk of the Town: Fashion Designers Uptown and Downtown ... The company's signature use of flowing fabric, with its softly crinkled look ...
www.fashionencyclopedia.com › Fr-Gu - Cached - Similar
Society Notebook: Walking the 'walk | The Portland Press Herald ...Apr 25, 2010 ... For painter and fashion designer Barbara Corey, drapey, flowing fabric is what catches her eye. She showed three pieces on the runway that ...
www.pressherald.com/life/.../walking-the-walk_2010-04-25.ht... - Cached

This is what immediately came to mind - and without looking at any of the other suggestions. Google 'floaty dress' or 'floaty blouse' and you get a lot of hits, including with refs to the 1920s if you include it, and it's certainly an expression I've used and heard plenty of times in the past.

I think this is what they're getting at with their 'vaporosi' - I don't thing 'gallegiare' can work/would work in the same way..... hence the use of vapour. And if you think about a floaty sleeve or hem - it can in fact look a bit like a vapour........