The Stack Overflow FAQ pageStack Overflow FAQ page states:
What kind of questions can I ask here?
if your question generally covers …
- […]
- software tools commonly used by programmers
- […]
… then you’re in the right place to ask your question!
Update (2015-02-06):
Since I originally wrote this answer, the Stack Overflow Help pagesStack Overflow Help pages have been changed, and now they state:
if your question generally covers…
- [...]
- software tools commonly used by programmers; and is
- a practical, answerable problem that is unique to software development
… then you’re in the right place to ask your question!
So questions about Vim are currently on-topic only if they are "unique to software development". So for example a question about cindent would be on-topic, but a question about autoindent might not be, depending on context, and a question about how macros work definitely would not be.
If you're looking for somewhere else to post your question, might I suggest the dedicated Vi and Vim Stack Exchange site, currently in public beta.