Your iTunes volume settings are stored in iTunes-only metadata called "iTunNORM." The universal standard is ReplayGain. They are measured a little bit differently but will both adjust playback for perceived loudness.
One difference is that you generally don't set ReplayGain values manually; you run ReplayGain analysis to have volume levels set for you.
To convert existing iTunNORM values to ReplayGain values using Mp3tag: There are multiple steps and considerations, so try this forum post: http://forums.mp3tag.de/lofiversion/index.php/t9139.html
To measure ReplayGain correctly: Use a ReplayGain scanner to update audio file metadata. (foobar2000 -- the greatest audio player ever invented by humans -- can do this for you as well.)
To use ReplayGain in VLC: In v2.0.5, go to Preferences > Audio > Replay gain mode, then select either Track or Album.
I suspect you want Track mode. Regarding Track-gain and album-gain (from Wikipedia again),
ReplayGain analysis can be performed on individual tracks, so that all tracks will be of equal volume on playback. Analysis can also be performed on a per-album basis. In album-gain analysis an additional peak-value and gain-value, which will be shared by the whole album, is calculated. Using the album-gain values during playback will preserve the volume differences among tracks on an album.
On playback, listeners may decide if they want all tracks to sound equally loud or if they want all albums to sound equally loud with different tracks having different loudness. In album-gain mode, when album-gain data is missing, players should use track-gain data instead.
If you still use iTunes, iPod, etc. you may also try finding some tools available to convert ReplayGain values back to iTunNORM. (A few are listed in the "Scanners" section of the Wikipedia links above.)