Update: the previously suggested --volume argument no longer works.
In more recent versions of VLC, you may adjust volume manually via command line parameter:
vlc.exe [file] --gain [value]
The value is documented in --help --advanced to be a float in range of [0.000000 .. 8.000000], but test show that higher, even much higher values are also accepted (tested up to few thousands).
The gain value is independent of volume value, e.g. regardless of the gain value and effective volume, the in-application volume will show as 100% or any other preset value.
Example:
vlc.exe %HOMEDIR%\Desktop\movie.flv --gain 8
