Is your vacuum system intact, but leaking, or completely messed up?

If its the first, don't panic, its likely not as bad as it looks. First, start with your engine bay. I suggest getting some new rubber hose (probably 3 meters or so), and either an assortment of cheap plastic vacuum fittings of the appropriate size, or buy the correct MB vacuum fittings (count what you have, buy those). The important thing is to be methodical. Take many more pictures of the vacuum system than you think you will need. Then take some more. Now you can start replacing hoses and fittings. Replace exactly ONE hose or fitting at a time. Keep doing this until you have no old hoses or fittings. Inspect your hard lines as you go. If you find a hole or crack, its fine to cut the hard line and splice it together with rubber hose. Typically your engine bay is the most degraded area.

Now if everything works, walk away and enjoy the frosty beverage of your choice. If not, its time to start troubleshooting. Invest in a hand held vacuum pump with a gauge and start looking for leaks. If your door lock system is disconnected from your engine, you can skip that for now. The first thing I would try is isolating the system to your HVAC. (On a w123 I believe it is a green/yellow line that passes through to the cabin. Try to pull a vacuum on that with the hand pump, if it holds, try cycling through all of the heat & air settings. This will help you narrow down what circuits are leaking. If you dont care about the hvac, just cap the line that leads to the system.

If your system is truly chopped up, I wish you the best of luck. I have not found a very clear diagram for my car, hopefully you can.