Which ingredient is NOT used in a traditional Vodka Martini?

In a traditional Vodka Martini, the primary ingredients are vodka and dry vermouth. The inclusion of dry vermouth is essential as it adds complexity and depth to the drink without overpowering the vodka. Olives are commonly used as a garnish for Vodka Martinis, contributing an additional flavor profile.

Bitters, while not a standard ingredient in a classic Vodka Martini, can be used in some variations but are not typical. However, using bitters does not significantly alter the core nature of the cocktail.

Triple sec, on the other hand, is a sweet orange-flavored liqueur. It is not an ingredient associated with the Vodka Martini, which aims to maintain a certain purity and simplicity by focusing on the vodka and vermouth. Thus, because Triple sec is not part of the traditional recipe, it is the correct choice for this question.