The youtube tag is now strongly discouraged. Instead, use the [noparse][video][/video][/noparse] tags. Not only does the video tag work just as well as the Youtube tag, it also works for other services, such as Google, Vimeo, Hulu, MetaCafe, and Facebook.

With the [noparse][video][/noparse] tag, place the entire link within the brackets, like so:

[noparse][video]http://www.youtube.com/watch?v=ZEdesU-LU24[/video][/noparse]

[video=youtube;ZEdesU-LU24]http://www.youtube.com/watch?v=ZEdesU-LU24[/video]

You'll notice that after you make the post, vBulletin will alter the tag to look like this --> [noparse][video=text;text]text[/video][/noparse] <-- the additional stuff put in there is generated by vBulletin and you don't have to add anything for it.