Brother of The Benz, EstebanSPI
Not having viewed every 300E, your situation sounds quite bazar.
First thing about the function of the vac resevior. I supplies vacuum for all of the items that function on vacuum when your engine, at that moment, is not providing adequate manifold vac remain in the positionuum. This would be when the engine is under a load or when you are driving at high elevation. It also allows those vacuum elements to operate smoothly and remain in the position directed by a controller (autmatic climate control).
Second and I have no answer except if you aren't the original owner, it could have had damage at the left front fender area.
I would suggest you try to find a used overflow tank. It doesn't have to be a M/B unit just one that would fit with little mods.(cheaper Too!)
Happy Trails Beep Beep from The Spiderman in Houston!!!