Is there any data on how many users are using Vi vs Vim? I'm under the impression that it's mostly Vim (you may prove me wrong though).
If this should be the go to destination for Vim users then IMHO the name should reflect this.
You're right, most questions are about Vim. But what if the site was just named "Vim Stack Exchange"? Then just by name it would suggest that only questions about Vim are allowed, and that all other questions would be off-topic (which they're not). Questions about original-vi or nvi are not common, but they do get asked.
Vim just so happens to be the most popular vi clone at this point in time. It has been for a while, but this can change in the future. Remember how fast Xorg replaced XFree? Or how fast svn replaced CVS, which was in turn replaced by git (and in lesser degree hg)? This can happen to Vim as well.
NeoVim doesn't have quite the uptick as those examples. There are probably several reasons for that, but that could change in the future. Or another project could take Vim's place as de-facto vi clone in the future.
IMHO "Vi and Vim" is the best name for now, and naming the site just "Vim" would be roughly analogous to renaming the Unix site to "Linux" just because that so happens to be the most popular Unix clone at the moment (or worse, "Debian" because that's the most popular Linux distro).
Previous related discussions:
vi
is some version of Vim.vi
when I want to do something right now and the whole secondvim
needs to load is too much for me :P