Also it helps to strip down the embed code that you get from the site and only use the embed key itself without termination.
The forum software will actually provide everything else.
For example the following clip.
The following is the embed code provided by youtube with the angle brackets replaced with curly brackets so that the code is not interpeted.
{object width="425" height="344"}{param name="movie" value="http://www.youtube.com/v/vWLfUvTfTHo&hl=en_US&fs=1&"}{/param}{param name="allowFullScreen" value="true"}{/param}{param name="allowscriptaccess" value="always"}{/param}{embed src="//www.youtube.com/v/vWLfUvTfTHo&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"}{/embed}{/object}
So all you do is "Edit HTML Source" and paste in the above and delete all but the following tag and the forum software will provide the rest. Nost that in this case the src= term appears first but that doesn't matter just copy the embed tag and the embed tag *only*.
{embed src="//www.youtube.com/v/vWLfUvTfTHo&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"}
One other caveat emptor is that it is sometimes dependent on the browser that you're using. At home this works for me with FF and not IE7 but at work it works with both. YMMV