The youtube tag is now strongly discouraged. Instead, use the [video][/video] 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 [video] tag, place the entire link within the brackets, like so:
You'll notice that after you make the post, vBulletin will alter the tag to look like this --> [video=text;text]text[/video] <-- the additional stuff put in there is generated by vBulletin and you don't have to add anything for it.