What is on-topic on Vi and Vim Stack Exchange that isn't on Stack Overflow? I don't get it: what's the point of this site? What could I ask here that I cannot ask on Stack Overflow, where there's already huge Vim knowledge present?
Stack Overflow is for professional and enthusiast programmers. If you take a quick look around, it will be clear that almost none of the questions here are programming-specific.
Vim is used by far more people than just programmers. Vim can be used by writers, mathematicians (LaTeX), sysadmins, students (for taking notes), or simply anyone who needs to edit text in general.
Many of the questions we have here would be explicitly off-topic for Stack Overflow, as they have nothing to do with programming.
Finally, the existence of one site does not justify the prevention of the creation of another one. A few prime examples: Ask Ubuntu vs. Unix & Linux and Mathematica vs. Stack Overflow. For both of these, one is a subset of the other, yet they have vastly different communities and would not work well together if merged.
Questions about vi but not about text editing: