Extensions did recently become a part of VLC, but there is currently very little information floating around about them. I do recall seeing a subtitles extension in the nightlies a while ago, but it didn't work at the time. Perhaps that's worth a look.
There are a couple of applications out there for this purpose:
- SubDownloader -- Open source, cross-platform but shareware on Windows (?!). Last I checked it didn't work, presumably due to an API change. When it did work it was pretty good.
- Sublight -- Windows only.
- SolEol -- Cross-platform, broken last I checked.
- FileBot -- Cross-platform
I haven't found a solution that I like yet, so I decided to slowly develop my own. Currently I use a combination of Google, OpenSubtitles and TVsubtitles. I recently tested FileBot and found it to work very well.