Refers to the broken English utilized by African slaves who learned to speak English during their enslavement in America. This form of speech became prefered by White people of the time, as it validated their belief that Africans were inferior and incapable of learning to be civilized. After a time, many children and decendants of slaves, both freed and still in captivity, utilized some form of slave dialect while in the presence of White people, but in private spoke proper English, read books, and wrote poetry.
An example of Slave Vernacular taken from Paul Lawrence Dunbar's (1872-1906)"An Easy Goin Feller":
"An' ez fur boss, I'll be my own,
I like to jest be let alone,
To plough my strip an' tend my bees,
An' do jest like I doggoned please."