You only need to put this part in the youtube tags: mNGvvk_E_xQ
Long explanation:
So, original url: /watch?v=mNGvvk_E_xQ&feature=player_embedded
All you want is the information after v=.
These are known as variable pairs. name=value. So you just need the "v" value, nothing else. The variable stops whenever you see a &, that signifies a new pair. In this case, feature=player_embedded
So all you need is the v value.
Edit: Wow just found another way to do it.
Just put the url in the box with no tags or anything around it.
[ame="http://www.youtube.com/watch?v=mNGvvk_E_xQ&feature=player_embedded"]The Time Is Now : Be Legendary - YouTube[/ame]