I have a fresh install of Windows 8 with a fresh install of 2.1.0. The --volume command line switch doesn't seem to work. For example, the following command will launch vlc with 100% volume:
"C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" --volume 0 -vvv "http://path.to/stream.pls"
No matter what value I enter for the volume, vlc launches with my volume setting from the last time I ran it. I also tried the latest 2.1.1 nightly with the same results.