echo 'set editing-mode vi' >>~/.inputrc
This will enable vi-editing mode for all commands that use the GNU readline library (e.g. bash, python, bc, etc.), not only the shell. For the shell only (including non-bash shells), add 'set -o vi' to your shrc file.

I don't know why I didn't do this before given that I'm a pretty hard vi user. Still, for some reason, I kept using emacs-like key bindings for command-line editing. Not any more! However, be careful: if you are used to vim's visual editing mode, you'll keep hitting 'v' in the command line and getting super annoyed.