To make a Vesper Martini with orange bitters, start by filling a mixing glass with ice. Then, add 3 ounces of London Dry Gin, 1 ounce of vodka, and 1/2 ounce of Lillet Blanc. Add 2 dashes of orange bitters to the mix. Stir everything well with a bar spoon for about 15-20 seconds until the mixture is well-chilled. Strain the drink into a chilled cocktail glass. Finally, garnish the cocktail with a lemon twist and enjoy.