Terms like vernacular architecture can have many meanings, so some defining seems in order. "Vernacular" is defined in the dictionary as characteristic of a period, place or group and this definition is often applied to some rural architecture which was created by builders or even owner/builders following established (if unwritten) principles. This meaning is similar to terms like "folk" architecture or even that furry term "traditional" architecture. These buildings often perform well functionally or they would have been discarded. One of the great examples - the log cabin - came from Sweden and spread throughout the US because it was easy to build with readily available materials and performed well in many climates.

Vernacular style in the city gets a little more confusing. Here it implies a lack of "high" style which is related to utilitarian architecture often found in warehouses or industrial buildings. Some small 18th and 19th century houses built as rental housing lack much in the way of decoration, but often share the symmetry and proportions of architecture of a defined style like Georgian. Are they undecorated Georgian or are they vernacular style?

Terms like vernacular and traditional architecture also have had a political life of their own. In the past, they were often used pejoratively meaning "uneducated" or "unsophisticated" and were contrasted with formal styles like Georgian or Greek Revival. As architects became more of a factor in the building industry, the vernacular/folk world was defined as buildings built without being designed by architects. In the 1960's all this was turned on its head when vernacular and folk architecture was seen as a purer and less affected architecture than "high" art architecture. A memorable study called "Architecture Without Architects: A Short Introduction to Non-Pedigreed Architecture" was published in 1965 and the populist title says it all.
The Vernacular Style
Swedish Cabin, Upper Darby, PA