By definition (at least according to Merriam-Webster) Vernacular is a noun meaning "of, relating to, or characteristic of a period, place, or group". And to a greater extent, vernacular is especially defined as "of, relating to, or being the common building style of a period or place". In essence, vernacular architecture is concerned with domestic and functional buildings rather than monumental and adorned buildings. It's the common domestic structure whose design is based largely on function (and even expression of structure) that resonates with the vernacular. So what does it mean to design a building rooted in the vernacular expression of form and materiality? It means as an architect I prefer to start a new project by designing a building's fundamental form based upon a regionally appropriate typology, and that the materials from which I imagine the structure will be constructed can be found regionally too. However, the intent is to build upon the vernacular form and elevate it to become something unique - its about creating a particular composition of components to differentiate present day designs from the buildings of years past.
(image from bobvila.com)