James Bond was an alcoholic, with good reason. He didn't drink vodka martinis for the taste, he drank them to dull the pain and horrors of his job. As much as he drank, he probably didn't really taste the booze anymore.
The original James Bond from the novels was a dark and brooding high-functioning alcoholic, who operated at his best with a drink or two in him at all times. He was pretty useless without the drink. A vodka martini would quickly get him in the right headspace to accomplish his latest mission.
The movie Bond was reinvented to be this dashing, handsome womanizer who drank and smoked socially and was charming as hell. Basically, a 1950s ideal male fantasy. This Bond probably could've used a classier drink than straight vodka, but that's one aspect of the books they kept pretty loyal.