Currently as it stands vBulletin (subsim forum application) does not support embedded YouTube as standard.
To get around this custom BB code is required which can only be installed via the admin panel (By Unkel Neal).
As can be seen here
http://www.geekpoint.net/threads/you...bulletin.3877/
The method I used as Manos linked doesn't actually embed the YouTube video.
Instead I cheated and used a static pic of the video cropped from the YouTube site.
This pic is then hyperlinked to the actual video. Crude but effective.