Vintage fashion is referred by which era or period a garment may come from. When someone says, "1950s" or "1970s" you are referring to an Era. The decade. In fashion, it is actually divided by periods. A period of time in which is distinctive characters specific those years.
Fashion is broken up into periods by these dates:
1947-1964 The New Look
1965-1976 Mod, Hippie, Disco
1977-1989 Punk, New Wave
1990-unspecified - Bondage, Goth
So many people sling the phrase around, "1950s dress" when actually you are referring to a period of time from 1947 to 1964 where poodle skirts and circle skirts were worn. Beaded cardigans and saddle shoes were hot. This period is defined by specific garments in an era.
Here is a visual example. Both dresses were produced in the 1960s. The party dress on the left dates from 1960 to 1964 but; is referred to as a "1950s dress" because of the period it defines. It is a characteristic of this time in fashion. The dress on the right is mid to late 1960s. It is defined loosely as "1960s" because of the period in fashion it comes from. As you see above in the fashion timeline the dress on the left is a classic "New Look" dress and the one on the left is a classic "Mod" style dress.