MLA does not have a specific recommendation for YouTube/Vimeo/Online videos, so the standard is to use the format for online film or video. Here are the basics:
Author’s Name or Poster’s Username. “Title of Video.” Online video clip. Name of Website. Name of Website’s Publisher, date of posting. Web. date retrieved.
It is very common for any web resources not to have all of the info MLA would like. If any piece is unavailable, make your best effort to find it. Sometimes you have to look at other portions of the website to find the required info, and sometimes it is truly unavailable. In that case, skip it and move on to the next piece required.
You could also try using EasyBib's form for Film and online video.
Here is an example (without the hanging indent):
Shimabukuro, Jake. "Ukulele Weeps by Jake Shimabukuro." Online video clip. YouTube. YouTube, 22 Apr. 2006. Web. 9 Sept. 2010.