Do you have a mod for embedding youtube videos? Maybe it's not respecting the code tags.
Code:
<iframe width="560" height="315" src="http://www.youtube.com/embed/8UVNT4wvIGY" frameborder="0" allowfullscreen></iframe>
It's not happening here or on my test site so I don't think it's standard vbulletin behavior. If you're not sure, try disabling all plugins by editing includes/config.php and putting this:
Code:
define('DISABLE_HOOKS', 1);
just under the <?php at the top.