I think others have done this or tried doing this, but it was very entertaining nevertheless! I haven't kicked it up to the next level of having the Vectrex run a dumb display loop and having the Arduino stuff the line data into the cartridge in real-time, but I would like to do that eventually!
Happy to share anything that I have. I made a Vectrex cartridge model in Eagle and I have schematics, etc. available.