I was looking at your Vectrex repair logs a few weeks back.
I had two to fix. One turned out to be a dodgy on-off switch, only one pole of the switch though so the power to the processor board was getting through (you could hear it play), but not the supply to the monitor. The other had a faulty PIA, it would just remain crashed from power up.
I did the opamp sound mod too, you are right, it is a bugger to do.