I think most would consider Cocchi Americano much closer than Lillet Blanc -- modern Lillet has basically no bitterness. But that's rather the point, and at some level, the challenge -- a cocktail is designed around and balanced around its ingredients, so when you change one, you have to rebalance it. For a showcase cocktail like a Vesper, I'd consider that a new cocktail -- in the same way that an Aperol Negroni is not a Negroni. And when making a non-showcase cocktail with spirits called out by family, your job is to balance around the selected ingredients. A Woodford + Carpano Antica Manhattan is not going to optimize at the same ratio as a Basil Haden + Dolin Rouge Manhattan, for example.