Don't do it, it's painful
We really don't want to start answering a question on a SE site just to see it was already answered elsewhere. It's better if we can collaborate to build one good answer rather than having parts of the answer made on different sites. A duplicate question brings nothing good.
Don't do it, it's mostly useless
Putting your question on SO or superuser already gives it a big audience. If what you want is to touch all people who are really interested in a given tag, then chances are that any site will do it because we have tools to be notified when a question appears in any site of the network, like email alerts or RSS feeds. So, simply choose the most relevant. And users are already looking at the sites holding the type of question they want to see and answer, no need to spam them elsewhere.
Regarding your specific question
In my opinion, this question should have been asked on superuser. There is no programming involved and it's only related to the use of the vim software. This doesn't mean it must be closed on SO, because as vim is commonly used for programming it might be seen as in our interest sphere but the most relevant SE site would have been superuser.