with the vectrex unplugged from mains socket, you could try turning the volume knob all the way up, then all the way down (not switched off though), then all the way up again, all the way down, repeat doing that about 30 times...
then when u try the vectrex again, see if the behaviour of the sound issue has changed at all
you could try reseating the soundchip if you are confident with removing and reinserting socketed chips (it's prob not the problem but if comfortable removing/reinserting chips, might as well try)
also as already mentioned, some switch cleaner spray in the on/off/volume pot, but as you may have already guessed, you have to take half the vectrex apart to get to it, unfortunately