Bring up Finder and press Shift-Command-. to make it display invisible files. In your home folder, you should see a folder called ".config". Navigate to ".config/VintagestoryData, and you will see a file called "clientsettings.json". Edit that file in your preferred text editor (TextEdit, VSCode, etc.) and look for the line:
"selectedBlockOutline": true,
It was line 67 in my file. Change the line to read:
"selectedBlockOutline": false,
Save the file, start Vintage Story, and you should be good to go. This fix worked for me.