Vernacular speech that is not accepted as suitable for formal usage is called Slang
Step-by-step explanation:
A vernacular, speech is the variety of language,that is used by an individual in everyday life.It is used by the people residing in a geographical or social location
The term Vernacular refers to the common language that is spoken by an average number of citizens of a particular geographic location, or it refers to the language used within a particular profession or industry.
An example of vernacular is the law terminologies used by the lawyers
Thus we can say that Vernacular speech that is not accepted as suitable for formal usage is called Slang