Email Client with Vim Editing

Try mutt. You can use external editor with that, default is vim. The navigation is all similar to vim too.

Use It's All Text! to make your text boxes (including the GMail plain text compose box) editable in an external editor. Or you can set $EDITOR to vim and use alpine.

I know the question was posted almost six years ago, but for anyone else who in the future might land here while trying to figure out how to get vim into every aspect of their lives, I recommend checking out the wasavi extension. Essentially, Ctrl-Enter will start a vi buffer within the textbox your cursor is currently in. Do all your vi magic and hit ZZ and you're done!

Works in Gmail (which was my primary use case when looking around for something like this, hence why I ended up here). In fact, this answer was typed using vi from within the textbox. Hopefully wasavi will meet your needs. I know I am absolutely loving it.