Plex/xbmc is a more of a home entertainment system. Vlc is merely a media player (that is incredibly awesome at what it does, thus the reputation).
I use xbmc on my TV in the front room because it tracks where I am in a series and which movies I've seen. I use vlc on my computer to watch a one off documentary or something.
I think you could build a plex or xbmc like system using vlc but to compare them I think is comparing apples to oranges.
For what its worth I stick to XBMC when I don't need a web interface. Plex isn't entirely Foss, xbmc is.
