I'm guessing the "vent" is a heat duct? Possibly the entire furnace is under the sofa? (I've owned two rigs like that.) The short answer is that you can re-route the duct, extend, shorten, make a turn or whatever suits your revised layout.
Capping off the duct, however, is not advised. The furnace needs to move some minimum volume of air through to avoid overheating itself.