The Vesper was released to the world in the 1953 Ian Flemming novel Casino Royale as "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."
Since 1953, three out of four of the drink's ingredients have changed. Gordon's gin is lower proof than it once was, as is most vodka you'll find on store shelves. The lemon peel probably didn't change too much.
Kina Lillet (a fortified aperitif wine, a distant relative of vermouth) became rebranded as Lillet in the 1980's and much speculation has been made about how its flavor changed. The alcoholic strength was lowered by a couple of degrees and the amount of sugar was lessened, but it has been generally assumed to be made far less bitter than it once was. (The word "kina" is a derivitive of "quinquina" which is a derivative of "quinine," the bittering agent also found in tonic water.)
However, at the world's largest cocktail convention, Tales of the Cocktail in New Orleans, the producers of Lillet opened a bottle of pre-formula change Kina Lillet for us to try. Its fruit flavors had dulled a bit over time as you'd expect from a fortified wine that old, and it did taste sweeter and just a touch more alcoholic. But most importantly to cocktail geeks it tasted more bitter - but really not all that much more.
Experts agree that in order to recreate the Vesper accurately, you should use high-proof gin and vodka. Many people recommend adding quinine powder to the Lillet, but perhaps in light of this tasting just a dash of bitters will do.
1 fl. oz. 100-Proof Vodka (50% alcohol)
3 fl. oz. 94-Proof Gin (47% alcohol)
.5 fl. oz. Lillet Blanc (There are now Rouge and Rose versions available)
1 Dash Angostura Orange Bitters
Stir all ingredients with ice and strain into a cocktail glass.