The most part of VWG is open source. There are some basic DLL's that are just distributed as DLL's and then there will be some commercial extended controls that will not be open source... that's the way I understand it.
In the download area, which you are probably referring to, there are only the prepackaged SDK installation files for the compiled version of the project. To get to the sources, you have to go through SVN. Info on how to get to the sources you can find in the developer area http://www.visualwebgui.com/Developers/tabid/360/Default.aspx and there you can for instance take a look at the Contribution guidelines so see how you get to the sources.
Also, there is a video in the video section: http://www.visualwebgui.com/Community/Resources/Videos/tabid/320/Default.aspx about working with the sources.
More info about this is probably all around the site, but these were just the few I came up with instantly :-)
Páll Björnsson -
Visual WebGui support team - Email: firstname.lastname@example.org