Currently in bash I use set -o vi to enable vi mode in my bash prompt.
How do I get this going in iPython?
In case someone's wandering in here recently, IPython 5.0 switched from readline to prompt_toolkit, so an updated answer to this question is to pass an option:
$ ipython --TerminalInteractiveShell.editing_mode=vi
... or to set it globally in the profile configuration (~/.ipython/profile_default/ipython_config.py; create it with ipython profile create if you don't have it) with:
c.TerminalInteractiveShell.editing_mode = 'vi'