It would be nice if the ones giving advice knew a bit more about how these ignition systems work. You can't mix up VR with VA - it simply won't work right.

If you have constant vacuum at idle that drops off to nothing by 1,500 RPM or less, then you have a throttle body VR system.

If you have no vacuum at idle that increases as RPM's increase, then you have VA.

Another system used on MB engines has constant manifold vacuum and the vacuum is shut off using a vacuum switch over valve controlled by a speed sensor.

The fact that the vacuum drops off a bit while bliping the throttle means nothing to the function of the distributor. This has some affect on a VA unit but it's not considerable.

The best way to determine which system you have is to hook up a vacuum pump or gauge to the various ports and see what signal you have.
I would also check the distributor to see which direction the point plate is moving. With rotation is VR and against rotation is VA.

Installing a pertonix unit will only replace the points and it will have no effect on the operation of the vacuum system. I disagree with the notion that a 190SL mechanical distribuitor won't work properly. The advance curve is slow enough that you shouldn't expirience any pinging.