None of the above.
Write a better answer.
Closing is only for questions that are unanswerable or that are off-topic. The fact that a question has been answered elsewhere is irrelevant.
Lack of research is a valid reason to downvote, but just because the question has been asked somewhere else on the Internet doesn't imply a lack of research. (For one thing, has this elsewhere question been answered satisfactorily?)
Copying the answer is a possibility¹, if there's an answer that's really great, but it shouldn't happen too often. The whole point for this site to exist is to be a better place for vi questions than SO. If SO is the place to go for vi answers, there's no point in having a site dedicated to vi.
Many vi answers on SO are somewhat minimalistic. Don't just copy an answer that says “press
kkjjhlhl21i”: explain why it works, link to the manual, suggest additional plugins that help with the task, discuss portability across vi implementations, etc.
Especially in the early beta, don't be content with adequate answers. Aim for awesome.
¹ You may always copy Stack Exchange answers, with attribution.