According to their official website:
VLSub is not working with Vlc 2.1.x on any platform because the lua
"net" module needed to interact with opensubtitles has been removed in
this release for the extensions. Works with Vlc 2.2 on mac and linux.
Too bad Ubuntu has 2.1.x. So, first step is to get a more recent version of VLC:
sudo add-apt-repository ppa:videolan/master-daily
sudo apt-get update
sudo apt-get install vlc
At the time of writing, the VLSub plugin in the Ubuntu repositories (0.9.10-1) does not search for subtitles, and the latest version from GitHub has problems opening the subtitle once downloaded. So, I created a fork that actually works (tested on VLC 2.2.0~~git20140609+r57027+123~ubuntu14.04.1 and all later releases, no problems so far):
To install locally, type:
cp vlsub.lua ~/.local/share/vlc/lua/extensions/
Or install system-wide:
sudo cp vlsub.lua /usr/lib/vlc/lua/extensions/
And you're done. Enjoy.