Ok, so Mamiya wrote me back:

I have checked on your question and can find no reason why the old Vivitar 225 flash should cause any problems for your camera. Part of the reasoning behind that is that Profoto strobes have a very high sync voltage and they do not cause any issues, even though they do not use the hot shoe. However, it may be a good idea to contact Vivitar directly as they may know something that we do not, just to be certain that there will be no issues for the camera.

Im pretty certain voltage is different when from hot shoe or sync input. Ive emailed Vivitar in the past and they never responded.

With all that said, I went and bought myself a nice Metz 20 C2

I'm sure it would be fine, but Id rather not test it and in the long term regret it.