I don't like these best-dressed lists from Vogue, because they're always wrong. I don't know exactly how they judge but it is not always the best choice. Once they said that Natalia Vodianova was the the best dressed, but there were another people from the list that were so much better dressed that her.
I think the staff just pick their favourites among the party/socialite crowd regardless of style (since undoubtedly half of them will be ODLR/Caroline Herrera fans) with a sprinkling of whatever actress got press this year that fits into the Vogue circle. In that sense Vogue is more Tatler than...well Vogue.