xcape runs as a daemon and intercepts the Control key. If the Control key is pressed and released on its own, it will generate an Escape key event.
This makes more sense if you have remapped your Caps Lock key to Control. Future versions of this program might do that mapping for you, but for now this is something that you have to do yourself.
If you don't understand why anybody would want this, I'm guessing that Vim is not your favourite text editor ;)
$ sudo apt-get install git gcc make libx11-dev libxtst-dev $ mkdir xcape $ cd xcape $ git clone https://github.com/alols/xcape.git . $ make
Find the latest version at https://github.com/alols/xcape
The author can be reached at albin dot olsson at gmail dot com