(defun edit-buffer-in-vi ()
  "Put the content of the buffer into an external file and invoke VI on it."
  (let ((filename (make-temp-name "silly.vi.user."))
	(message  (current-buffer)))
    (with-temp-buffer filename
		      (insert (buffer-string message)))
    (let ((editor (start-process-shell-command "message editor"
					       nil ; no associated buffer...
					       "xterm"
					       "-e"
					       "vim"
					       filename)))
      (cons editor filename))))

(defun edit-buffer-in-vi-make-closure (buffer filename)
  "Restore the content of buffer from filename when invoked."
  (let ((buffer-name (buffer-name buffer))) ; Oh for closures...
    `(lambda ()
      (let ((buffer (get-buffer ,buffer-name)))
	(when buffer
	  (with-current-buffer buffer
	    (erase-buffer)
	    (insert-file-contents ,filename)))))))
  
(add-hook 'message-setup-hook		; am I the right hook?
	  (lambda ()
	    "Invoke an external editor on the content of this buffer..."
	    (let ((result (edit-buffer-in-vi)))
	      (set-process-sentinel
	       (car result)
	       (edit-buffer-in-vi-make-closure (current-buffer) (cdr result))))))