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Readline vi Mode
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While the Readline library does not have a full set of `vi' editing
functions, it does contain enough to allow simple editing of the line.
The Readline `vi' mode behaves as specified in the POSIX 1003.2
standard.
In order to switch interactively between `emacs' and `vi' editing
modes, use the command M-C-j (toggle-editing-mode). The Readline
default is `emacs' mode.
When you enter a line in `vi' mode, you are already placed in
`insertion' mode, as if you had typed an `i'. Pressing <ESC> switches
you into `command' mode, where you can edit the text of the line with
the standard `vi' movement keys, move to previous history lines with
`k' and subsequent lines with `j', and so forth.