I agree with @josh-petrie, except for the following points.
TL;DR: Allow the question on vi.SE if the user is relatively new to SE itself -- this site is the most apropos place to ask. And also allow it if it is similar but not fully answered elsewhere (SO).
(A) Say a user new to SE overall has a question about Vim. They are more likely to search on this site, for the fact that it's the site's focus. Now have the user write a question. No similar questions appear on the right. They will think it is "new" to the entire stack, and continue to post it. In this case, we should answer it best we can in purely Vim terms... attempting to write an answer different than what is on SO.
(B) Similar to above... Say someone familiar with SE searches for a question first on SO. It helps them out 95%. That other 5% is still crucial. At first glance, the new question on vi.SE will look the same. BUT DO NOT CROSS-LINK IMMEDIATELY! You take the time to read it, and find a way to answer it with a different perspective, a vi perspective.