Double clicking a file while downloading will not cause a file to become corrupt. In windows though VLC will normally not play a file by double-clicking it in Limewire.

Some players will play the file when double-clicking, but this is not the best option. If the file is downloading while you try to play it the player will most likely not be able to render the file. Pausing and then double clicking often works and no duplicate preview file will be created when doing it like that.

On the other hand fixing the file using VLC may very well cause corruption. Choosing the fix option will allow VLC to process the file as it wants, rewriting headers etc. In most cases limewire will not recognise the file after doing this.

If you can't preview a file and want to fix it:

Go to the incomplete folder, right-click the file, choose copy. Then paste wherever you like (outside of the incomplete folder of course), and fix that one instead. Then after viewing just delete the copy.

It is pretty uncommon that VLC won't be able to play an incomplete file without fixing it, but it does happen.