You want to compose an encrypted mail message, and your mail editor is vim.
~/.vimrc: map ^E :1,$!gpg --armor --encrypt 2>/dev/null^M^L map ^G :1,$!gpg --armor --encrypt --sign 2>/dev/null^M^L map ^Y :1,$!gpg --clearsign 2>/dev/null^M^L
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These macros filter the entire edit buffer (1,$) through gpg. The first macro merely encrypts the buffer, the second encrypts and signs, and the third only signs. You'll be prompted for your passphrase for any signing.
gpg(1), vim(1). Credit goes to Rick van Rein for this tip: http://rick.vanrein.org/linux/tricks/elmPGP.html.