I don't worry that a bear will invade my tent and eat me if I were to cook in the vestibule. It's more that I don't want to attract a bear to my campsite for any reason. Over and above that, I don't want a bear investigating my tent for food whether I am in it or not. There's no good can come of that, even if what does come of it would certainly fall far short of my death at the paws and jaws of a bloodthirsty bruin.