bit horrible to have bator without vent opened, imagine you're inside that bator and with so little amount of oxygen and the CO2 keep growing as you're breathing, soon the oxygen will be changed with CO2 and soon you'll feel lost of power and unconscious and in few hours..... all is lost.
that's why you must have at least one or two vents opened to make sure the bator have good air exchange, fan only *rotating* the air inside bator to make sure there's no hot/cold spot inside bator, fan can't produce oxygen.
i guess the vent is located at the side of bator, that will be good to have vents located at side of bator, not the top since it will make sure the temp/humid not fluctuate too much, if the vents located at top, it will be hard to keep the temp/humid stable since the law is hot air rise.