I have two iMacs both at macOS 10.14.6
On one of them, I downloaded the latest version of the VASSAL ENGINE, intending to upgrade it from 3.2.12 - the other is already at 3.2.17 from a few months ago.
Anyway, when I launched the .dmg file I got an error...
VASSAL-3.2.17-macosx no mountable file systems (see image).
I checked on my other system and got the same error (even though that machine was already at 3.2.17)
I downloaded archived versions and ran those on both machines - same error.
I attempted to run other .dmg files from other providers - these all worked, without error.
My conclusion is that either the vassal engines .dmg files are corrupt or not compatible with this version of macos.
Can anyone else repeat my findings?