Saying “shaken, not stirred” signals both know-how and toughness
Lowell Edmunds, a Harvard-educated Classics professor who has also written exhaustively about the martini, says that the way Bond orders his cocktails speaks to a “toughness and deliberate insensitivity.” In essence, as gin was thought to be a delicate, bruise-able drink, to order one with vodka was to reject the daintier spirit.
Gatewood went on to say that ordering a drink this way displays that “[Bond] has discriminating ability -- connoisseurship -- as well as knowledge of the usual way a bartender would make the drink." Bond is indicating that he’s a veteran drinker, and like anyone who is an expert of a discipline's rules, knows how to break them. An order that brash can come from ignorance or absolute mastery -- if you're talking about a man with a license to kill, we'll lean towards the latter.