I guess its "anonymous" in the sense that the authorities couldn't quickly identify posters in order to disrupt gatherings immediately. However, I don't believe that there's any long-term anonymity (e.g. if you incite a deadly riot by falsely yelling "bomb", the authorities will come looking for you). If the police really wanted to find out who posted something, they could subpoena Vibe for information about the phone (e.g. phone number) that made the anonymous post.

Perhaps Vibe destroys all data about users/posts as soon as the posts expire...but even then, the cell networks would have some record.