vacuum

So I am pretty sure I have a bad vacuum leak, but I am having trouble isolating it. It is also possible I could have a different problem that presents as a vacuum leak. I am stumped on how to proceed. The car is a 1998 Mustang GT with the 2v 4.6L. It was driving fine, but then had two problems...

Does anybody know what this vacuum hose goes to (it just disappears in the fender) and if it is mission critical? It comes in to a port right behind the throttle body. The hose is pretty old and porous. I'm on the hunt for vacuum leaks and I'd like to unplug it to test if this is the culprit...