You don’t need to have Java installed separately–VASSAL will no longer use system-installed Java anyway, it comes with its own. Although you didn’t specify what CPU your Mac has, this very much sounds like you have a newer Apple Silicon (M1 or M2) processor, so it’s possible you mistakenly installed the MacOS build for Intel CPUs. See this recent thread–if you have an M1 CPU and the correct build installed already, there’s a preference you want to enable if for some reason it wasn’t by default.