Your mysterious stray vacuum line could be connected to the air cleaner or the breather hose from the valve cover which leads to the air cleaner. Look at both of these (probably off of the car right now) for the possible answer.
I had a similar issue with my V8, see the following thread.
Tip for V8 Owners
By the way, the device on my car which had the vacuum line looked just like yours and was located right by the intake, just like yours. My vacuum hose was a plastic 3/16" hose, unlike your hose which extends to the left in your picture. I don't know what the device is either, some kind of fuel pressure regulator?