The vernacular language is the language of the country, the language spoken by 'ordinary people' (and acquired as their mother tongue). In much of Europe it also meant 'as opposed to Latin'. Vernacular literature in Europe is used for literature in the language of the country (again, by contrast with literature in a foreign language). So, in the Middle Ages, Middle High German literature was vernacular, for example.
In England, the vernacular meant 'English as opposed to Latin and French'.