Often times a vintage collector will be called out for wearing "costumes". And to the average eye vintage clothing may look like costumes. The styling is different from what most people are use to but vintage is far from costume.
Speaking for myself, I don't consider vintage to be a costume. I live in 2010, own a computer (obviously), drive a vehicle less than ten years old and don't speak like an iGent. I'm modern throughout except for my taste in clothing because, quite frankly, modern clothing is terrible. I prefer the quality, styles and cut that vintage has to offer.
A few vintage collectors may fancy themselves as living in the 1930s and therefore wear a costume, but most do not.
So what exactly is a costume?
In part, it has to do with mindset. Like those few 'vintagers' who believe they are truly living in the Golden Era, costumes (the people wearing them) believe or at the very least act like they are something they are not. In order for there to be a costume there first must be something that is make believe. And mindset is the gateway through which something becomes an act.
On the other side of the viewing glass, the viewer's mindset is just as important as the wearer's. Movies and false notions of the Golden Era have shaped the viewer's mind into thinking that anything vintage looking or inspired by vintage is a costume: a fedora (any fedora) gets the ubiquitous Indiana Jones remark; a pinstripe double breasted suit becomes a gangster suit; full cut trousers transform into a zoot suit, etc. Rather than trying to learn about and understand clothing from the Golden Era and the folks to collect it, most Joes use their ignorance of the period as a crutch.
Physically, a costume is false. On the outside it looks pretty good, like the real deal. But inside, down to the details it is ugly. Costumes are thrown together with budget and ease of production in mind. Halloween costumes are flimsy. Most stage costumes are pieced together from what can be found.
And most importantly, costumes don't fit right. They're made to create a certain appearance and nothing more.
Here's where I might ruffle some feathers.
Rather than 'vintagers' being the ones wearing costumes, I argue that iGents and other folks who wear ill-fitting modern pieces are really the ones who wear costumes.
These are the trademarks of modern costumes: low armholes, paper-thin fabrics, horrible fits and cuts made by machines (off-the-rack) and even by a surprising number of modern tailors (made-to-measure and bespoke), and a super high price tag. All one must do to find such costumes is search the numberous online clothing fora.
So rather than seeing a different clothing style as a costume, first look and see if it meets the above trademarks of a costume. And if it does, rest assured that it's the emperor's new clothes.