Sipsmith Gin,very Juniper forward, and Absolut Wheat Vodka. Then, because we have no Kina type aperitif and taking inspiration from Gabriella14's query below we used 1/6th Lillet Blanc and 1/6th Suze; this produced a delightful dry, herby but clean drink with the most beautiful yellowy hue. Wonderful.
Also, Vespers is a Church of England early evening service, usually around six o'clock; it is not inconceivable that Ian Fleming, as an expat Englishman, enjoyed the joke when telling his guests it "was time for Vespers!" and then serving his chosen Cocktail.