What is vernacular speech that not accepted as suitable for highly formal usage but much used in conversation?

You're describing colloquialisms. These are words, phrases, or expressions that are characteristic of informal speech and writing, often specific to a particular region or social group. They're generally considered inappropriate for highly formal settings like academic writing or official speeches.

Here are some examples:

* "Gonna" instead of "going to"

* "Y'all" instead of "you all"

* "Ain't" instead of "am not" or "are not"

* "Catch you later" instead of "goodbye"

* "Hit the road" instead of "leave"

Colloquialisms can be quite common and even preferred in informal settings. They can add a sense of familiarity, informality, and even humor to conversations. However, it's important to be aware of their limitations and choose your language appropriately based on the context and audience.