Question on vacuum hose size..
 

I am wondering if there are supposed to be two different sizes of vacuum line around the engine, because there already is in my car. I never noticed until one broke, that it is much larger than many others around it. And I didn't think it odd until going to order more line of that size, when all there is on every MB parts site [I know of] are the small 4mm size lines. I've got tons of this small diameter line, is it okay to use anywhere firewall forward?

Yes, there seems to be skinny and thick types.

I have these rubber lines that are the same size as the larger ones, would it still function the same or does it HAVE to be the traditional hard plastic?

The rubber lines will work as long as they don't collapse. They will take up more room and probably won't last as long. If you've got a bunch of rubber hose sitting around, might as well use it.

Jeremy