Vassal is a great way to try out a game, but it seems that vassal versions and module versions frequently create trouble actually trying something out, i.e. you want to try something that requires the latest version, so you remove your older version and install the latest version. Then it will no longer open a module written for a lower version.
As vassal is Java (I think) it probably doesn't use the windows registry, but it could have some favorite directories it stores things in. If different versions use the same installed program GUID under windows, that could cause some serious pain. There are lots of ways things could go wrong with multiple installations, some of which might not be obvious right after installation.
So what is the most painless way to keep multiple versions of vassal around in windows so that they won't step on each other's toes? I don't really want to run one VM per vassal installation.
Any other way of solving problem that different modules need different vassal installations on windows is just as good, as that is the root problem.