Newer versions of VLC no longer contain the teletext decoder as part of the base install package.
If you are watching a DVB-T stream and when you try to turn subtitles on you get an error message about 'telx' codec not available then you need to get and install the 'vlc-plugin-zvbi' package. Use your favourite package manager or open a terminal window and enter the following command:
- sudo apt-get install vlc-plugin-zvbi
Restart VLC and subtitles should now be working.