The module
VFB 0.1 [www.vassal40k.com] version 0.1
contains custom code that was compiled with Java 1.6. This will fail on a system that only has Java 1.5 installed with the error
VASSAL.build.IllegalBuildException: java.lang.UnsupportedClassVersionError: Bad version number in .class file
Vassal is built with Java 1.5 to maintain compatibility with older systems. Any custom code should do so also.
B.