How can I use vim not vi to write commit message?

From the editor docs:

Mercurial tries to pick which program
to call to edit a commit message by
trying the following (in order):

1. HGEDITOR environment variable
2. editor configuration option in [ui] section (in hgrc or passed with
   –config ui.editor command-line option).
3. VISUAL environment variable
4. EDITOR environment variable
5. vi, if none of the above is set

Personally I prefer to just do hg commit -m "My commit message"