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I had the same problem and I added the following lines to my vimrc to fix the movement issues.

noremap j gj

noremap k gk

This is not flesh out, but it does support vim keybindings. http://ivanceras.github.io/spongedown-editor/

Nice, I tried Spongedown but it seems to have at least 0.5 or 1 sec delay between typing and rendering, so it wouldn't be really usable for me.