1. (of language) native or indigenous (opposed to literary or learned ).
2. expressed or written in the native language of a place, as literary works
3. using plain, everyday, ordinary language.
1. the native speech or language of a place.
2. the language or vocabulary peculiar to a class or profession.
3. a vernacular word or expression.
4. the plain variety of language in everyday use by ordinary people.
While Sacred songs are often sung in Latin, Secular tunes are always sung in the vernacular as their primary purpose is to entertain the masses.
I ended up taking out some of the definitions listed. I found them to be a bit confusing, and honestly, the definitions I provide here are how I've always heard the word used. Enjoy!