According to JustJaredJr.com, Vanessa Hudgens has NOT auditioned for 'New Moon'.
Vanessa's rep tells JustJared, “I don’t know anything about the audition, as far as I know, it’s just a rumor.”
SpoilerTV even posted a list of Casting Calls for New Moon and the character of Leah is not being cast for. As we all know, Leah's role is really minor in New Moon. It doesn't get big until the last chapters of Eclipse.