What is the main component of a cocktail described as a woodford bourbon-based drink with vanilla hints?

The primary component of the cocktail described is indeed the Old Fashioned. This classic cocktail is characterized by its use of bourbon, and in particular, Woodford Reserve Bourbon is known for its rich, full flavor profile that can include notes of vanilla, caramel, and oak. The Old Fashioned traditionally emphasizes the flavors of the spirit used, and in this case, the vanilla hints align well with the characteristics of Woodford Bourbon.

The other options do not fit the description as closely. The Dragonfruit Cosmo is a fruity, colorful cocktail that incorporates flavors like cranberry and dragonfruit, which do not involve bourbon or vanilla in a significant way. A Pomegranate Margarita is centered around tequila and pomegranate flavors, straying even further from the bourbon foundation. The Crown Apple Mule includes Crown Royal Apple whisky and ginger beer, focusing on apple and spice notes rather than bourbon and vanilla.

This context clarifies why the Old Fashioned is the correct choice, as it directly aligns with the specified base spirit and flavor profiles present in the question.