posted by Misca .
Which is grammatically correct? I'm not really sure how onomatopoeias work.
1. “Vroom!” the chainsaw took right off.
2. "Vroom!" The chainsaw took right off.
Number 2 is grammatically correct. Since you have an end mark -- an exclamation point -- after "Vroom," the next sentence should start with a capital letter.
so it's ok that "Vroom!" is a fragment?
Yes, "Vroom!" is a fragment, but it's accepted in informal writing.