Sure.
You see, when you are watching a video on Youtube, the video itself gets temporary stored on your harddrive while having the tab/site open. hehe
Now, the method I use on Opera browser, is:
In Opera, the cache is stored at default in win 7 is,
C:\Users\------your name?------\AppData\Local\Opera\Opera\cache
Make sure that you sort by Date modified/edited in this folder. (in sub folders, sort by size)
Now go to a video in Youtube and let it load. while it is loading, you go back to the cache folder and right-click, update.
The folder who is modified, or a folder that just popped up, look into that/those folder(s), and look for the growing .tmp file.
A good way find it more quickly , is to sort by size (biggest on top)
When the video is fully loaded, copy the file, paste it somewhere, right-click, rename, remove .tmp at the end and write .flv
oh, make sure you show the extensions to the file-types.
Now double-click and enjoy it on VLC media player.