I'd like to be able to switch temporarily from emacs mode to vi mode, since vi mode is sometimes better, but I'm usually half-way through typing something before I realize I want

I don't want to switch permanently to vi mode, because I normally prefer emacs mode on the command line, mostly because it's what I'm used to, and over the years many of the keystrokes have become second nature. (As an editor I generally use emacs in viper mode, so that I can use both vi and emacs keystrokes, since I found myself accidentally using them in vi all the time, and screwing things up, and because in some cases I find vi keystrokes more memorable and handy, and in other cases emacs.)