Deleting the configuration files for vectrex does allows the line to work. However, it appears if you change the overlay manually though the in-game configuration menu, it saves the setting to vectrex.cfg. Then that overlay will appear again, even if you use the -view command.
So, as it works right now, you need to delete all vectrex configuration files (vectrex.ini, vectrex.cfg), and use the -view command:
mess.exe vectrex -cart C:\Emulator\Mess\Software\Vectrex\Cartridge\Armor.zip -view "Armor Attack overlay"
However, do not change or select any overlay manually, or else that overlay is saved to the configuration file vectrex.cfg.
-Trebor