I assume you entered the URL correctly (no https but http, and only the v=... part, like
Open the Extras menu and click on Messages. Now click play again and see what message's there.
My guess is it's
lua error: Couldn't extract youtube video URL, please check for updates to this script
VLC parses YouTube's HTML source code to look for the video, but sometimes, YouTube changes its way it outputs the HTML, so VLC is unable to find the video URL.
- Go to https://www.youtube.com/html5 and enable html5 there to play the video in your browser without flash
minitube and try to play the video there
- Wait for an update of VLC (or update the lua script yourself - not so easy, see Updating the VLC YouTube parser)
- Install the package
youtube-dl and download the video to your harddisk. It's a command line utility though
Updating the VLC YouTube parser
Update just the Lua script for YouTube parsing to the latest version from VLC:
Open the Terminal application and enter these commands:
curl is installed, we need this to download the new file
sudo apt-get install curl
Get rid of the old, non-working scripts
sudo rm /usr/lib/vlc/lua/playlist/youtube.*
Download the current ones (use the whole command line, yes, it's quite long):
sudo curl "http://git.videolan.org/?p=vlc.git;a=blob_plain;f=share/lua/playlist/youtube.lua;hb=HEAD" -o /usr/lib/vlc/lua/playlist/youtube.lua
See if it works.