VLC 2.1.5 - unable to find file caching values in preferences
Solution 1:
I'm not familiar with VLC's streaming features, and I'm running 2.0.5 not 2.1.5, but I believe this is what you are looking for.
Go to "media", "stream", the tab "Network", click "Show more options", and choose your desired caching amount either using the caching form or the edit options.
Solution 2:
You can config caching on Tools -> preferences (show settings "All") -> Input / Codecs. In Advanced option, define your preferences for caching.