Using vernacular is a useful and powerful tool adding to characterization, setting, mood, narrative style and voice.
But how much is too much?
Vernacular can be effectively expressed with very little change from formal speech patterns.
How do you know if your vernacular is too heavy?
If your reader has to work too hard to understand what's being said, you're obviously using too much. If most of your vernacular comes from a plethora of apostrophes, you're definitely using too much. If, in reading your story aloud, you begin channeling Jeff Foxworthy, you're probably using too much.
Simple sentence in a formal speech pattern:
"A man content to go to heaven alone will never go to heaven."
With vernacular (American Southern, for this example):
"If you're happy to go to your glory without your darlin' you ain't going to get there any time soon."
That's just fine. I've changed it to a vernacular by carefully choosing the words I use, the order in which I place them, with a very few obviously vernacularized words--like ain't and darlin'. It could probably do with just a little bit more, but only when used as dialogue. Not generally as narrative. If you have a story that requires heavy vernacular, you'll want it to be in dialogue, and either VERY toned down or nonexistent in the narrative. There should be a distinct difference between the narrative and the dialogue. Narrative should always be somewhat more formal that dialogue, which should be less formal in order to make it sound natural. Vernacular--at any level--can be hard to read for more than a few pages. So a bit more:
"If you wanna go to yer glory alone, glory ain't gonna want you."
That's not TOO over the top, but a lot of editors will be annoyed by it, just because a lot of editors get annoyed by vernacular. Be aware of that. Every choice you make in your story will change its possibilities in the world of publishing, because editors, like everyone else, have biases.
Now to really hit it with the kind of vernacular that's going to result in a big fat rejection, guaranteed:
"If'n yer thinkin' on goin' te yer glow-ry all solitaree-like, ye ain't gwyna be nun too pleezed wit' whar ye ind up."
Ugh. And yes, I've read stories like that. Ugh.
The purpose of vernacular is NOT to accurately portray an accent. Your readers' imaginations will fill in the accent if you give them a very few spare clues as to what that accent should be.
The key to getting a vernacular right is to know it. Listen to it. Go to youtube and listen to Russian speech patterns, German speech patterns, Scottish speech patterns, African speech patterns, Asian speech patterns, Australian speech patterns, etc. Don't try to emulate them EXACTLY in your writing, but give just enough that the reader can imagine it.
After all, that's the most important job your writing has to do--to engage your reader's imagination.