A ‘Vesper’ is a cocktail drink that was introduced in Ian Fleming’s novel ‘Casino Royale’. John Gruber is a fan of the James Bond stories and of high alcoholic beverages. I don’t see how the name has anything to do with note-taking, though.
Like Vesper, the name ‘Q Branch’ also refers to the James Bond franchise (Q is the mastermind who provides James Bond with all of his clever gadgets).
http://www.macworld.com/article/2040883/meet-vesper-a-notes-...