“A dry martini,” [Bond] said. “One. In a deep champagne goblet.”
“Oui, monsieur.”
“Just a moment. Three measures of Gordon’s, one of vodka, half a measure of Kina Lillet. Shake it very well until it’s ice-cold, then add a large thin slice of lemon peel. Got it?”
“Certainly, monsieur.” The barman seemed pleased with the idea.
“Gosh, that’s certainly a drink,” said Leiter.
Bond laughed. “When I’m … er … concentrating,” he explained, “I never have more than one drink before dinner. But I do like that one to be large and very strong and very cold and very well-made.” – Ian Fleming, Casino Royale, Chapter 7, “Rouge et Noir’

There’s a problem for anyone attempting to follow this original recipe, as alert reader Brad Frank pointed out when I first posted it several months ago on my own Facebook page. Kina Lillet, a fortified wine from Bordeaux that contained quinine, no longer exists in the form Fleming knew it. Most recipes today substitute Lillet Blanc, but that version of the aperitif lacks the quinine Fleming would have known. Brad assures me the taste of the resulting Vesper is entirely different from Bond’s version. He, along with numerous liquor industry websites, recommends using Cocchi Americano instead for a more accurate drink. 

There’s an excellent demonstration of a Vesper right here:
http://video.epicurious.com/watch/how-to-make-a-vesper-cocktail. I like that fact that the mixologist behind the bar discusses Fleming as he shakes. Notice that he uses Lillet Blanc regardless of the change in recipe. Many people do; but lacking quinine’s bitter edge, the drink is more a Vespa than a Vesper.

Eva Green as Vesper Lynd in the 2006 film of Casino Royale

Bond names the drink after his Service colleague in Casino Royale, Vesper Lynd; and it’s probable that Fleming was making an ironic point with Bond’s gesture. Vesper is strong, intoxicating, and goes down smooth–but she’s got a depth of bitterness that lingers in the aftertaste. Bond’s affection for this gin-dominant cocktail endures only as long as the lady in question. In later books Bond shifts to a pure vodka martini–shaken, not stirred. 

So it seems that the Vesper today is very much what one decides to make it. Some mixologists suggest adding bitters to the Lillet Blanc. Others substitute dry vermouth for the Lillet altogether. One ardent scribe settles for nothing less than adding a piece of cinchona bark (the original source of quinine) to the Lillet, then uses it after the quinine has been released into the aperitif.

If you can find cinchona, I salute you.
In the meantime, go for the Vespa–a cold and heady summer martini without the bitter finish.

Cheers,
Francine