VASL runs as a plugin to the VASSAL board game engine. You must install the VASSAL engine before you can install VASL.
Download the VASSAL engine from the VASSAL.org website: http://www.vassalengine.org/.
For download and installation instructions for the VASSAL Engine, see the VASSAL Users Guide at https://obj.vassalengine.org/images/8/8c/Userguide.pdf