Yes you are correct.
The file name is unimportant for the cfg file.
The png file name MUST match the overlay0_overlay = in the cfg as you stated above.
So to load the Berzek one
Drop the Berzerk png and cfg into your overlays folder
GO To Settings -> Onscreen Overlay
Display Overlay = ON
Overlay Preset = (Navigate to and select the Berzerk.cfg file)
Those are the steps for loading an overlay
BUT
If you do what I have just explained this will load the Berzerk overlay for EVERY game you load with the Vectrex core.
To get it to load different Overlays for eaxh game you will need to do exactly the same thing AND also create a per game override that will change the .cfg file retroarch will look for when loading the overlay.
I dont fully know how to do that so cant help you with that step
The rough process IF you are using a recent Nightlie versions is built into the RetroArch menus
- Set up Berzerk as above
- Load up a different game
- Go and change the Overlay Preset = option to the overlay .cfg for the new game
- Choose the option Save game Override (May be named different IDK)
Now do that for all your games