I've designed an add-on device for my VecAdapt controller adapters. Now Atari compatible joysticks can be used with the Vectrex as well as the Mega Drive / Genesis controllers that the VecAdapt works with directly.
To make up for the lack of buttons on the Atari joysticks, a mode is provided where Up/Down movements trigger Vectrex button functions, which can be configured using the buttons on the device.
I've been playing with mine. Of course it is best with joystick games that only require one action button. But I also found it useable and interesting in some multi-button games even including setups involving switching the configuration during gameplay.
Something cool to consider and I think that should be advertised more clearly was a surprise to me is that you can assign ANY vectrex button to the Atari button WITHOUT forfeiting up/down on the joystick. The two swap buttons on the new device work even when the "up/down to fire" button is not pressed. So pressing 3/4 swap makes the Atari fire button 3. Then flipping the button swap on the VecAdapt makes 1 or 2 the fire button, depending on if the 1/2 swap is pressed. You can have the full four direction joystick plus any single Vectrex button.
I'm not sure if anyone would design a new joystick around this, but it's a nice option that you could now based on the straight-forward-as-could-be design of Atari digital controllers. For example a joystick just for War of the Worlds that had a restricted Left/Right joystick and then arcade buttons for Atari up and down. I guess maximum available Vectrex buttons at a time without reconfiguring would be three so I'd eliminate the pause button in WotW.
I tried stringing in line a third adapter I have that is for adapting 15pin analog PC sticks to digital Atari. It did not work unfortunately.
Thanks for the comments. I'll have a go at revising the description at some point to highlight the Fire button assignment. Maybe a diagram, if I can design one that reduces confusion rather than adding to it.
Shame about the PC joystick adapter not working, I can imagine what the problem is (it's outputs don't work quite the same as physical switches), but there's not much to be done about it.