the youtube tag method has been deprecated (by vBulletin)) in favour of
[video=video-source;video-link]full-url-to-video[/video]
video-source is the provider site , i.e. youtube, vimeo, etc
video-link is everything after the watch?v= in a youtube link and similar for other sources
full-url-to-video is the full link to the video as appears in the address bar e.g. https://www.youtube.com/watch?v=adsiengfh (that is a nonsense link)
so a full example (based on the url used above) would be
[MEDIA=youtube]adsiengfh[/MEDIA]