Vine users are being alerted of a bug that apparently exposed their e-mail addresses and phone numbers to third parties. Parent Twitter is informing all users who may be affected by the same and is also asking them to be on the look-out for any e-mails from unknown senders, as this may have the potential for further damage.
Twitter has refused to go into much detail, however, we do know for certain that the company was not hacked. It was a bug which led to a situation wherein the data belonging to Vine users was only available under a very specific set of conditions.
We are basically dealing in probabilities here. There was a point o time where this user data was vulnerable and someone could have used an API to access it remotely. However, there are no indications that someone was actually aware of the issue and managed to exploit it.
Here is the letter all affected Vine users received (Via TechCrunch):
Meanwhile, all that Vine users should know at this point is that their accounts are safe, they do not need to change their passwords and that any official communication coming their way will come from an @twitter.com email address.