Why are the words in the vocabularies of chapter two listed as
vita, -ae?
Is that because the first declension are those words forming their genitive singular in -ae, or another reason, or none at all?
Nouns in the vocabulary lists (in both Greek and Latin) appear with the nominative (lexical) form and the genitive. This is extremely useful since it will indicate to you at a glace which declension a noun falls into. You should learn both forms so that you remember how it will look in other cases.
A word of caution, Wheelock also lists adjectives with -nom -gen in the first couple of chapters, but dictionaries and further chapters use a different method later.
Thanks for that, I would have been confused otherwise.