There is a very unofficial poll here: http://sixrevisions.com/web-development/the-15-most-popular-text-editors-for-developers/ FWIW, I think the niche where Vim dominates the competition is that it's the only editor that reliably works through a command-line ssh session. I would bet that anyone who has had to administer a remote system through SSH is comfortable with, or at least familiar with Vim, with the possible exception of Dr. Richard Hipp who I think uses ed, or possibly a GPL-licensed clone of Edlin. Vim and Emacs also seem to have modes for the newest programming languages before other editors.
You may also find some information on the Editor War Wikipedia page, maybe on the talk page or in the references. Some people search job boards and sites like StackExchange and count the hits for various technologies like emacs vs. vi or vim (emacs emacs emacs bwahahahah!).
Finally, someone once told me that if I was really serious about using Emacs everywhere, I needed to know enough Vi to download and build Emacs on any operating system. Which is to say that Vi comes pre-installed with most flavors of *nix and most *nix emulators. From personal experience I can tell you that the Alt key has taken on special significance in windowed-desktops a way that makes emacs difficult or impossible to use in a gui environment when started from a terminal. Vim has no such difficulties.
P.S. For the record, I now use Emacs and vim almost equally. Emacs when I want to use elisp and regular expressions to format text in a buffer (e.g. to format output of sql queries or command line scripts for use in spreadsheets). Vim for any general file editing from the terminal or through ssh.