I don't know all the details of the W126 system or the system you bought, but the basic idea is simple. When you lock/unlock your drivers door, the rest of the doors follow. There's no reason to change that if it works. What you need to do is install the electric actuator that will make the drivers door lock/unlock just like using the key or pushing the lock plunger. The actuator is usually a couple of inches long and about 1/2 inch in diameter. The bottom end has some screw holes so it can be attached to the inner door frame (you may have to drill a hole or two). The upper end has a small rod that you need to attach to the door lock rod with a small clamp. The result of the actuator is the same as pushing or pulling the door lock plunger in the car. The actuator is wired to the keyless system such that it pushes/pulls in the correct direction.

I don't know if you need to do any additional wiring to operate the vacuum pump in the trunk. If you sit in the car with the engine off (key removed) and manually lock the drivers door by pushing the plunger down, do the rest of the doors automatically lock? Do they automatically unlock if you wait a while and manually unlock the drivers door? If that works, I would say that you can leave the vacuum pump alone and just worry about making the drivers door lock work. Clear as mud?