You have a choice of "post editors", plain and fancy, in your profile. Make sure you have the fanciest "WYSIWYG" one selected to get the blue videocam icon. (It's kind of buried at the bottom of "Options").
Or just say "Frack it" and do it by hand -- wrap the YOUTUBE_VIDEOID tags around the Youtube ID (not the whole URL, just the ID number). Same as any vBulletin tag pair -- square brackets, open tag, the video-id, closing tag with a slash.
If I use parentheses instead of square brackets so you can see what it looks like, you get this:
(YOUTUBE_VIDEOID)-xyz1234(/YOUTUBE_VIDEOID)
Do not put the entire "embed" string from Youtube between the tags -- that won't work. You only want the part that identifies the video. We do the rest.