The MLA has recently released it's advice regarding citing a tweet. They suggest something like this:
Athar, Sohaib (ReallyVirtual). “Helicopter hovering above Abbottabad at 1AM (is a rare event).” 1 May 2011, 3:58 p.m. Tweet.
The APA blog gives some advice concerning the treatment of Twitter and Facebook. Their take looks like this:
BarackObama. (2009a, July 15). Launched American Graduation Initiative to help additional 5 mill. Americans graduate college by 2020: http://bit.ly/gcTX7 [Twitter post]. Retrieved from http://twitter.com/BarackObama/status/2651151366
And the Chicago Manual of Style offers this advice, and this example:
32. Garrett Kiely, Twitter post, September 14, 2011, 8:50 a.m., http://twitter.com/gkiely.
Now, Harvard isn't technically a style, but rather a system, and the version of Harvard we use at JCU isn't connected to a manual with regular website updates in response to user questions, but rather a hard copy style manual, which hasn't been updated since before Twitter was a thing. However, if you note the important pieces of information singled out by all the other styles (actual name, twitter username, contents of tweet, time and date, URL), then you can probably take the basic pattern of our version of Harvard and come up with something like this:
Fry, S (stephenfry) 2012, 'Fraudsters can access your details anywhere, even if, like me, you’re on the other side of the world http://bit.ly/wjMYnd #DevilsDetails', 13 March 2012, viewed 15 March 2012, <http://twitter.com/#!/stephenfry>.
As for Vancouver? Well, the odds that you will ever need to cite a tweet in any of the subjects that use Vancouver are slim-to-none, BUT - in the interest of fairness we'll have a go at that, too. This isn't the official version, but rather our mock-up following the pattern for blogs (as well as pulling from the styles for emails and discussion lists):
1. Fry, S. 'Fraudsters can access your details anywhere, even if, like me, you’re on the other side of the world http://bit.ly/wjMYnd #DevilsDetails' 2012 Mar 13 [cited 2012 Mar 15]. In: Twitter [Internet]. San Fransico: Twitter, Inc. c.2006- . Available from: http://twitter.com/#!/stephenfry.
Now, the referencing advice we give you always comes with two extra pieces of advice: 1) Be Consistent, and 2) Confirm With Your Lecturer. Whatever you do, do it the same way every time, and always follow the advice of the person who will be marking your paper.