Just to understand - did you change the setting System settings → Privacy & Security → Security to something other than "App Store and identified developers.”?
If you did, and you tried to re-install and run Vassal, and it still didn’t work, then you will most likely need to use the Alternative solution. It is not as scary as you may think at a first glance.
You can also try
- Open a Terminal
- Type in the command
and press enter.$ xattr -d com.apple.quarantine /Applications/VASSAL.app
Voila - that should remove quarantine flag and you should be able to execute Vassal.
Yours,
Christian