Question

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.
3. other?

Answer 1

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.

Answer 2

so it's ok that "Vroom!" is a fragment?

Answer 3

Yes, "Vroom!" is a fragment, but it's accepted in informal writing.