I was under the evident misapprehension that it was only on waistcoats of the english (not the french) style that it was deemed a faux pas to button the bottom button, because of the obesity of a certain english royal. After he became so rotund as to be unable to button his waistcoat all the way down, his court all wore the last button undone, so as to ingratiate themselves with, and spare the offense of, the corpulent king. That said, it has been a long time since I took a History class, so I am probably muddling my apocrypha.