escape-if-next-char-is-k.el

(defun viper-escape-if-next-char (c)
  "Watches the next letter.  If c, then switch to viper mode, otherwise insert a j and forward unpressed key to unread-command-events"
  (self-insert-command 1)
  (let ((next-key (read-event)))
    (if (= c next-key)
        (progn
          (delete-backward-char 1)
          (viper-mode))
      (setq unread-command-events (list next-key)))))

(defun viper-escape-if-next-char-is-k (arg)
  (interactive "p")
  (if (= arg 1)
      (viper-escape-if-next-char ?k)
    (self-insert-command arg)))

(define-key viper-insert-basic-map (kbd "j") 'viper-escape-if-next-char-is-k)