This is something I've a little experience with as I have dealt with bad vacuum modulators beginning with my first,a '63 Mercury Meteor.

Tranny fluid is almost pure white and very acrid when it burns,that's one clue.
Also dependant upon the inside diameter of the line the smoke will be either greater or less.

Additionally in certain speed ranges and shift positions you will be able to tell when it will happen,while in drive the tranny may have greater pressure thus seeming to make more smoke.

I do not know where the modulator is on your car but the very fact you have fluid in your vacuum lines would incline me to look there first,unfortunately some Mercedes,notably my '98 did away with a tranny dip stick,how about yours?

Ohh,has the brake fluid gone down?

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