JAMMA:
SPEAKER(-) L 10 SPEAKER (+)
MVS:
SPEAKER(Left) L 10 SPEAKER (Right)
That's what I think would be the problem... the L on a JAMMA is a ground, so on an MVS it would short.
I've heard it you can run the MVS on a Jamma with no problems, though. You could, very easily, by just wiring the speakers differently. Don't hook anything up to the L on the MVS side, or, just wire up a separate speaker
I've run my six slot on a JAMMA harness no problem, for testing. Of course, this was with a known bad sound board, so I knew it wouldn't cause any other damage
Of course, this is completely aside from the fact you need more buttons for an MVS