This is a popular thing to do, not only for vim users. ubuntu 13.10 removed xmodmap though, so now you have to use xkb to remap keys.
This is the command to remap caps-lock to escape:
setxkbmap -option caps:escape
The only way I found to make it permanent is to add the command as an autostart application.
- type "startup" in the unity dash
- choose "Startup Applications" and add an entry there
- choose name, comment and icon as you like, the command should be
setxkbmap -option caps:escape though.
After a logout/login caps should behave as escape now.