In the 90's, I bought a used VFX that had the keyboard problem as well as a torn ribbon cable that connected to
the Cartridge port. Though it was used, the dealer I bought it from managed to talk Ensoniq into a warranty
repair. We shipped it to Ensoniq, three weeks later it came back with every problem the same. I took it home
and repaired it myself cutting an 1/8" off the ribbon cable, and then I re-attached it to the socket. Worked fine
ever since.
However, before you get to far into attempting a repair, maybe you could find a ROM cartridge on Ebay to test
your cartridge port. The ROM cartridge can easily be converted to be an EEPROM cartridge. Instructions for
doing this are on this site, I believe. You do have to be able to solder.
Just a couple of ideas. Good luck.