Advantages:
Some advantages to writing in the vernacular are that first, it creates a vivid writing style, that helps the characters speak. Another thing it does is help show the time period of setting that the writing takes place in when you use unique vocabulary and or idioms. Another thing you might want to add is that "In The Vernacular" is just another way of saying that you are wring in the way most people talk to that it is easier to understand.
Hope this helps!