vk_msg_prunk.js

// to be executed in dev console with opened dialog on vk.com/im

var m = document.getElementsByClassName("im_msg_text"); 
for (var i = 0; i < m.length; i++) {
  m[i].innerHTML = decodeURI("%D0%AD%D1%82%D0%BE%20%D0%BA%D0%BE%D0%BD%D0%B5%D1%87%D0%BD%D0%BE%20%D0%BF%D0%B8%D0%B7%D0%B4%D0%B5%D1%86,%20%D0%B8%20%D0%B2%D1%8B%20%D0%BC%D0%B5%D0%BD%D1%8F%20%D0%BF%D0%BE%D1%80%D1%8F%D0%B4%D0%BA%D0%BE%D0%BC%20%D0%B7%D0%B0%D0%B5%D0%B1%D0%B0%D0%BB%D0%B8,%20%D0%BD%D0%BE%20") + m[i].innerHTML
};