This flavored martini is one of the easiest ways to enjoy a sweet vanilla flavor in a neat drink. The Natural Vanilla Martini gets its flavoring from a vanilla liqueur (originally it was Navan, which is now discontinued).
When you find a great vanilla spirit and pair it with a smooth vodka such as Belvedere, a wonderful cocktail is born. The lime juice is included to offset the sweetness and add a touch of citrus depth which is necessary and a welcome splash of flavor.
Tuaca, Bols Vanilla, and Galliano are all viable vanilla liqueurs and each will make a great Vanilla Martini. You might also try Brinley Gold Shipwreck Vanilla Rum, it's flavor is surprisingly reminiscent of what Navan offered.