I didn't have any luck slipping off the hose, but my friend helped me increase the slack of the hose by unclipping the hose from the coolant tank.
Giving the hose some more slack so that it could stand perpendicular to the vacuum tank seems to have temporarily taken care of the problem. The loud hissing noise is gone when the car is idling.
However, I noticed that the hose still hisses, albeit much less than before, after the car's turned off. I'm going to wait for my indy to come back and check it out further. Thanks again for the help!