There is an accessory vac leak traced to the A/C cabinet switchover valves. I plan on replacing the 5 valves.
Are the valves accessible from the passenger side dash underside kick plate removed?

There is a black vac line from the engine compartment routed from the engine switchover valves to a three way connector, one to the VCV bottom, and the other into the passenger compartment. Schematic labels it Vent to Passenger Compartment. It is open.
What does it connect to and is it accessible without removing the dash panel?

Advice on proceedures would be appreciated.

Although I do not have that exact model (anymore) I did carefully
disassemble one. If you are talking about the 5 or so vacuum switches/valves mounted to the heater plenum, then I seem to remember them being
directly behind the cabin climate control panel.
If that is true, then I am pretty almost sure that the center console should be removed
and the climate control panel removed to get at the bank of switches.

Not really as difficult as it feels. But can take a couple hours to do the
whole thing. I drove without a center console for a few days while I worked on it.

Regarding black lines that go from under hood to cabin area that are unplugged/open - my understanding is that these lines are open to atmosphere and do not connect to anything.
Removing the lower panels exposes them. Not sure if I understood the question.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

Agree electric/vac switchover valves. They shouldn't be too difficult to access.

The black vac line into the passenger compartment is only a "vent". I pulled the instrument cluster and bottom panel drivers side off and verified.
What is the logic behind a vacuum "vent"?
Venting vacuum to the lower valve of the VCV makes its vent function inoperative. Mvac test from the line out of the engine switchover valve results in zero vac three way hookup. A vacuum gauge reading from the three way black line out results in zero vac engine running. If the guage is the end point, not vented, 2 inches vac at idle and 5 inches with rpm around 2000. It's the way the switchover valves work. The purpose of vacuum attach lower VCV is to cause an abrupt drop in vac to the transmission modulator valve causing the transmission shift to be crisp.
An open vac will also cause less vacuum for the auxiliary functions such as doors, climate, engine shutdown.
MB knows best? That's why the car is a vacuum nightmare.
I removed the 3 way and have a vac line direct to the VCV lower valve. The car has enough vacuum leaks without intentional.

If you put vacuum in the lower part of the VCV, you will have too much vacuum going to the modulator. I dont remember if there is a T in there. My car doesnt have it. It shouldnt be a problem getting clean air if everything is working right. Thats the reason why its routed inside the cabin. Fix the leaks and everthing will be alright. It would be a good idea to remove the 3/2 valve on top of your valve cover if you still have them.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

The engine switchover valve line to VCV lower port is separate from the direct modulated line to the upper VCV port and to the modulator. Has nothing to do with getting clean air.