When you check-in to SVN with svn ci, the default editor opens to allow you to enter your commit message. My default editor was Emacs. I don’t use Emacs for anything else, and I found entering a simple message a tad frustrating as Emacs commands/shortcuts are different to what I use with Vim.
Enough is enough, time to change.
So, to change the default editor to Vim, simply enter the following command into your ~/.bash_rc or ~/.bash_profile file, then close and reopen your bash window.
export SVN_EDITOR=vim
Note: Here is a comprehensive guide to basic SVN commands which among other things, shows you how to set up SVN to use TextWrangler as your default editor – https://weblion.psu.edu/trac/weblion/wiki/SubversionBasics
magp.ie 
Although setting ENV is nice but I personally prefer changing the config file for svn.
in ~/.subversion/config file under [helpers]
you can set:
editor-cmd = vim
You can read the comments there for other options or info.